Could be a failing power supply, since it takes several attempts before it starts and runs. What color is the power button when it fails to start, and is it solid or blinking? If you use a power strip, surge protector or UPS, remove those and try connecting PC directly to the wall outlet.
Could also be failing RAM, failing hard drive or even a corrupted video card driver. Recommend you back up your personal files on external media asap in case it is a failing hard drive.
Suggest you reboot and press F12 before Windows starts to load. Go to Utilities partition and run all the hardware diagnostics, especially extended hard drive and RAM tests. Make note of any error messages.