Illustrator relinking files not working, invalid link — cannot read file.

Hi there, 

I'm having an issue that whenever my computer rests for a while, it restarts and then my illustrator documents unlink random pngs that have been in the file previously. The pngs haven't been moved or renamed or anything, but when I open it and try to relink there, Illustrator stops responding. Then when I open it and try to relink in the links panel, nothing happens. Then if I try to just place that same file in, I get a dialog box that says "invalid link, cannot read file." Any idea how to rememdy this?

Thanks!