I had the same problem.
Unplug the PC. Hold the power button for 30 seconds. Plug the computer back on. Turn it on again.
If it doesn't work:
Remove the small button battery from the motherboard and then proceed the start up in the normal way. Select F2 if the option appears, altho' the start up may go all the way normally.
Purchase NEW battery. First battery purchase replacement was a dud for me which really put me back trying all sorts of other stuff.
Please disconnect power to pc before removing the battery. If you got fat fingers use an electricians screwdriver to pop the battery holding clip.