What happened to Hodor?

Hodur (also called Hodr, Hod, and Hoder) is the blind Norse god of Winter and Darkness. He is extremely obscure, because our only knowledge of him comes from one episode in Norse mythology, which is known as "The Death of Balder". In fact, Hodur was one of the many sons of Odin and the twin brother of Balder.
hma.wikia.com/wiki/Hodur_%28Norse_God%29

Har!

I don't know, but I really like the theory that he was scared by something in the crypts. I remember that either in AGoT or ACoK there was a passage where Bran wanted to go to the crypts, but Hodor refused to obey him (although he had to enter them at a later point).

Other than that, I've once read one not seriously meant theory that Hodor is the real Aegon, whose head had been smashed by the door when his saviours had slammed it at Gregor Clegane. The injury to his head negatively effected his growth and intellectual development, and "Hodor" is in fact the truncation of "Hold the door!"  (OMG, the 5th episode is called The Door, isn't it? LOL)

Brandon's squire was imprisoned iirc. He was a Glover. I don't remember what happened to the others, I imagine they died though. 
I'm looking forward to finding out more on this. So many possibilities. It could be related to what happens at Winterfell during the Rebellion. Maybe something that explains why Benjen joined the NW? I did get the impression Bloodraven already knew the answer to Ryan's question though. Maybe Bloodraven tried to influence things and damaged Hodor so he would be left behind during the Wot5Ks to give Bran a means of travelling North? /crackpot. Sorry for cracking pots, been reading a series with a prescient character who does this kind of thing so very much on my mind 

My first thought was that Walder / Willis becoming Hodor will be the result of him and Benjen going to train together secretly, although they have explicitly been told not to do so. There could be a terrible accident, leaving W. severely injured / handicapped and thus becoming the Hodor we know.

Also was thinking that maybe feelings of guilt from this event could have led Benjen to join the Nights Watch (the reason for which GRRM refused to tell us), but as that was many years later it does not seem too plausible.

Great topic. After the latest episode, I'm thinking that whatever happened is not just an interesting fact that would round out Hodor's character but something central to the plot. In fact, I think it has to be some kind of major aha moment. There really isn't any other reason to focus on it in a flashback. If all they wanted was to show Lyanna as a young girl or that Jon was like Ned, there wouldn't have been the focus on what happened to Hodor, specifically.

While several of the theories you guys bring up are interesting, I don't see how any of them could lead to a big aha moment. Unless there is more to Benjen than we know. Like, whereever Benjen is and whatever he is doing has major significance.

I also think whatever happened to Hodor definitely damaged his brain. If he was just suffering mental or emotional angish, wouldn't Bran have learned all kinds of stuff while inside his head? He didn't.

Hrm, I'd forgotten the exact description. That could suggest some sort of mental and not physical ailment, I guess.

Hmm i think it might be a combination of both. This is more intriguing than the circumstances of jon's resurrection