HP Laptop Boots to Dreaded Blinking Cursor Screen

I see that lots of other people have problems with PC’s booting only to the blinking cursor screen. Mine started doing this last week after working very well but then one day it wouldn’t shut down. I pressed the power button for 5 sec to shut it off. Next morning it would only boot to BC screen. I reseated memory, tried new hard drive, reformatted the old drive – problem is I can’t boot from the CDROM either. I changed boot order in BIOS, etc – but nothing works. I have decided it is a bad MOBO or processor but before buying parts would like a second opinion. The laptop is too nice for me to replace the whole thing – I would love to fix this. Thanks for any help.