So this problem has just been occurring. I'd turn on my PC, it'd show me my ASUS first screen with DEL to enter set up, then the BIOS screen, then it would normally boot Windows 7. But instead I get a black screen with a blinking underscore at the top left corner. Same happens when I press DEL to enter the BIOS set up, the underscore shows. After about 6 times trying to boot and reboot, it finally booted.l This is the second time it has happened. My friend says it might be the HDD dying, but I still have no clue.

Any ideas what can or does cause this?

Usually the blinking under score is associated with, with BIOS failing to find a bootable device. So it could be a HDD, or you could have the drive in the wrong mode, but that would prevent booting altogether. That or an error in the boot order, but if nothing has changed, or been changed by you, then hardware failure is a possibility.