In the lizardmen codex it says that the Old Ones came from a different planet and that they brought up the races of Men, Elves and Dwarfs. I have heard it said that the they are a link between the 40k universe and this one. What do you think? If you think that it is what race do you think they where? and please which ever one you choose can you tell me Why.

They were Tyranids. They used Bio-morphs to create the races of the "Old World" to resemble the races they had already encountered in the 40k universe.

The Old Ones have appeared in GWs histories of both the 40k & Fantasy universes since their inception in the mid 80s.

If you look around you can find odd mentions here and ther of the Old Ones as a semi mythical master race in both 40k and WFB, remember that for some time both universe were linked and a lot of fluff crossed over, the Old Ones being one of the few bits of lore that still exists more in less in the same form in both universes.

From a WFB angle, one of the best indepth pieces of writing about the Old Ones is in the 1st edition Warhammer Fantasy Roleplay book, where they were called 'Old Slann'.

(the Lizardman army as it's known now has evolved significantly since the late 80s. First the Old Slann were the god-like Old Ones, then the general 'slann' race came to be an army (& blood bowl team!) withlizardmen and troglodyte unit choices, eventually lizardmen became the ainstay of the army with the near extinct slann becoming the leaders)

The 'Old Slann'/Old Ones introduced much life to the warhammer world and gnentically/magically created or manipulated the races of Elves, Dwarves, Humans and Halflings (and Ogre and more).

It was the collapse of the Old Ones' great gates that allowed Chaos into the world.

In 40k the Old Ones influence is less well documented but little mentions pop up in Eldar lore, the history of the C'tan and the Necrontyr, and elsewhere. I think they're mentioned or hinted at in the novel Legion in the sections about the cabal.

The sudden appearance of/rapid evolution of the Tau has been linked to the Old Ones, especially as parallels can be drawn between the Tau resistance to chaos, and the same in the WFB Halflings (also immune to chaos), one of the Old Ones last creations in that universe.

So, to answer your question, they weren't a 40k race that stumbled upon the Warhammer world, rather they are something far more vague and interesting.

In 40k the Old Ones were and ancient race that lived WELL before humans, necrontyr came along and were jealous of the old one's long live so they waged war on them. To help with the war the Old Ones CREATED (sound familiar?) the orks as the hordes of "foot troopers" and the Eldar with the ability to partially control the warp.
After the Old Ones became mainly wiped out and the Necrontyr dormant, the Eldar were sort of left to govern the galaxy, which they made a hash of (que the rise of the imperium)
I thought that the Old Ones of WFB were maybe Eldar, as at the point the Eldar were ruling, it talks of them learning "how to turn barren, lifeless planets into suitable planets to live on." which I think the Old Ones of WFB did to that world (correct me if I'm wrong). The Eldar also had their webway gates which could correspond to the Old Ones' Gate. In the rulebook it mentions "whether the Old Ones fled before the disaster" and hints "Perhaps the destruction of the gate was caused by the Old Ones themselves, to forstall an invasion" sounds like something the Eldar would do to save their Webway
But that's just my opinion.

At the beginning, there did seem to be a link between the two universes. WFB had boltguns and power armour and shuriken pistols, the Albion campaign(?) introduced them. However, since then, it has apparently been stated that the two worlds are utterly unconnected in any sense. They turn on the same ideas, using the same races etc. but beyond that nothing. The worst thing is that, for the life of me, I have no idea where I heard that. Not a clue.
Actually, having said the above, the Liber Chaotica is set in the WFB world but the writer has visions of the future in which he describes the Chaos Marines of 40k. So there may, yet, be a link to be revealed.

GFP

This just gave me a good idea, to write up a 40k campaign that takes place in the Empire. Orks raid an out of the way world (the world of Warhammer Fantasy) and begin to swiftly dominate, when some SM come to fight off the Orks in the vicinity of the Empire. Maybe add in something about CSM and the Eldar engaging their counterparts. Ultimate f***ed up crossover!!

They are the link and they are a race of their own.

The Old Ones are one of the few remaining links between Fantasy and 40k. I remember somewhere someones idea that Sigmar was a primarch.

that would make a lot of sense. whoevers idea that was deserves rep

Maby after the necrons made the old ones retreat they started up the world of the fanticy world. Soon afer the Chaos gods had to retreat mainly to the Fanticy world which is why the chaos gods have more of a role in fanticy than 40k (ex. eye of the gods) but kept there hold on 40k as well. The old ones knew this that was why they where so keen on making a magic resistant races. Then the Chaos gods came and forst the old ones to retreat even farther into space or where all killed of.

Do you think Old Ones where just really really good at magic or where gods???

I just thought what is warp stones/meteoroids could it be stuff from the necrons gods and not the Chaos gods???

Another thing I heard somewhere that the Chaos gods where old one but i was also told they wheren't

Just a thought