Assuming XP :
Start the PC from cold.
Press the power button
Keep tapping the f8 button on keyboard
a screen should appear with start up options
choose Safe mode
see if the PC gets to the Safe Mode desktop, if it does that's good as you haven't a major problem.
Close down, wait ofr a minute or two and restart using same procedure but choose Last Known Good option.
Hopefully this will restore your PC, otherwise come back to us here.