If it's not finding an OS, the first thing I would do is make sure you didn't unplug your HDD when you were installing your new PSU. Make sure the SATA data cable is plugged in to the motherboard and the drive. Check the SATA power cable to the drive as well.
Then try to boot up. If it doesn't get to Windows, have a look in the BIOS and see if the drive is showing in there. If it isn't then something is wrong with the drive. If it appears, but it doesn't load Windows then something is wrong with the file structure on Windows. If this is the case you may have to attempt a repair assuming you have either a Windows Repair disk or the Windows install disk.