Next time you start your computer, poke often, (not hold) the F8 key.
Eventually the boot choices menu screen will appear. One choice will be to disable automatic restart on error. Use the cursor keys and move the highlight up/down to that and then press the enter key.
Your computer will try again to start, and probably fail. There will be a blue screen remaining with a statement that it did not start successfully.
There will be a message that looks something like:
Inaccessible_boot_ device STOP 0x00000007B (0000000000x0, 00000000000x0, 0000000000x0)
In the above example, I show Inaccessible_boot_ device and a lot of zeros but your screen will show it's own error and a mix of numbers and letters.
Write down the error and all the STOP numbers, letters and come back with that information.