iMac middle 2010 and macbook pro early 2011 won't boot after mountain lion

I have a iMac 2010 and a MacBook Pro 13 early 2011 and had update them both to Mountain Lion (ML). After the second week of ML installed con the iMac the computer restarted by itself and stayed on the gray boot screen and did not booted. I verified and repaird disk and permisions but it did not work. I tried to boot safe mode but it did not booted. The only solution was to clean install mountain lion again. This happened 2 weeks ago on both computers and today on the iMac and I want to know if I coud get help to boot the computer again without reformat the computer.