Without knowing what you actualyl do, what goes on in your comp and so on nobody can tell you anything. From damaged footage to specific effects going bonkers to the endless AE disk cache bugs this could be anything. To salvage your project start by enabling capslock to completely suppress rendering and then disable al lthose potentiall offending things - check your effects, flush the caches etc..
I fixed the situation by importing it into another project.
Prior to that I could not check the effects, as I mentioned already. The message would not let me access my file, I click ok and the message pops up again in less than half a second, had to force shutdown to open Ae again and could not get in my file prior to the fix.