We have to know the error Number code or descriptive name for more accurate answers but a common issue after a power outage is damaged disk sectors. You can try running a CHKDSK /R booting from a repair disk, installation disk, or from a bootable live CD such as Hiren's Boot CD. You could also see if it can boot in Safe Mode pressing F8 immediately after the BIOS post... Or in case the bootloader suffered damage, you can also try a startup repair from a Windows 7 Repair or installation disk. The startup repair can also be available from the Safe Mode options list.