Wait, how did you run diag? From the bios? Or with an external utility?I've run diagnostics,no error.
If you can access the BIOS setup, then there can't be anything seriously wrong. If you have a windows setup file, load it onto a USB flash drive and boot up from it, and reinstall.
If you are lucky, you might even be able to launch startup repair from the USB and solve the issue. That rarely works though, so I wouldn't count on that. Reinstallation might be the best option.