|I think when you say "CPU" you are referring to Windows starting or the computer POST sequence starting up?|
Because the CPU is actually a small hardware component inside the computer
Some user's mistakenly call the computer box the Hard Drive or CPU
So following that information, I think you are saying that the computer does not POST (Power On Self Test)
But it did show the POST screen before?
Remove all power (power cord and main battery for laptops)
Hold the ON button in for 20secs (dispersing internal voltage)
Plug the power cord back in, and then turn on
If that doesn't work you may have loose connections inside the box or it could even be dust
You will need to open the side cover up and remove all internal dust and reconnect all internal connections
Then test again.
I hope this helps :)