Can't boot in2 ubuntu 10.04! HELP!?

i dual booted win7 n ubuntu10.04 a couple of months ago usin wubi, had no problems at all! until earlier 2day... i upgraded 2 software recommendations ubuntu made n was asked 2 restart, i did. now i can't boot in2 ubuntu! i'm asked 2 pick which OS 2 boot in2 @ startin screen, i pick ubuntu n startin screen comes on again, go through this loop ovr n ovr. PLZ HELP ASAP!