My rear fixed-gear wheel seized up yesterday on my commute home. I suspect it's the result of my bone-headed adjustment after replacing a broken locknut on the axle (I may not have put the new locknut on tight enough). The wheel now refuses to turn - the axle is bound.
I'm curious though: what has actually happened inside? I noticed it happen over one or two revolutions, as if something was wrapping around the wheel. Could it be that a bearing cage came loose and wrapped around the axle somehow? I've tried to take it apart and see, but though I got the locknuts off, I've nearly rounded off the cones themselves (and ruined a cone wrench) trying to get them unscrewed.
It is rare for a bike bearing to suddenly seize. Even a bearing "run dry" or exposed to saltwater or whatever will generally just get rough and squeaky.
In order to seize you generally have one of two things:
(Rare) A fragment of something -- sand, a sliver of metal, etc -- gets in between the balls in the bearing, causing them to lock.
(Common) The failure to properly fasten a lock nut or other fastener results in one of the bearing cones being "screwed" more and more tightly into the bearing, eventually making the bearing too tight to turn.
The good news is that the above second case rarely results in "fatal" damage (though it may have the effect of adding a few thousand miles to bearing wear). So long as the situation is corrected and the bearing not "forced" to ride a considerable distance the damage is usually minor.