You might have set the option to overclock the PC on startup which causes the problem, if you have integrated graphics (like most Intel CPU's have) then remove the 780ti from your PC, try to start it, when you get in windows delete the program you used to overclock the GPU, then put the 780ti back in and see if it works. You can also try to start in windows SAFE mode and delete the program you used for oveclocking that way.