Not too much info to go on... however, if you boot your MacBook Pro 9,1 with a Lion disk prior to 10.7.4 it will not boot. It will, however, boot from versions of Lion 10.7.4, 10.7.5, and of course Mountain Lion.
Ah. Is there any legitimate way for me to get a 10.7.5 installer? Or will Apple force me to do the .torrent route? I'll bet that's the issue.