the computer tells you the error with beeps
but beep standards are different to different motherboard manufacturers.
you need to google your motherboard company (or perhaps bios model I think) beep guide.
beeps mean encoded certain specific error, the length of beeps encodes which one
example: LOOONG BEEP, SHOOOORT BEEEP, LOOOONG BEEEP mean one error
LOONG BEEP, LOOONG BEEP, LOOONG BEEP mean another type of error described in the guide.