Sometimes when I turn on my PC, the Black Screen with blinking underscore shows up. Anyway I can fix this?

check all your connections are sound especially to the drives, and try resetting the bios, if your on a desktop pop the battery out for ten seconds and that may well work

That's a good idea. Might even try replacing the CMOS battery on the motherboard. I'm suspecting a wonky hard drive...but it could be a BIOS/CMOS issue. If you're able to get it started without replacing the CMOS battery, check the time on the computer. If it's off then the CMOS battery needs to be replaced.