Hi Mr P, and
I think someone should be able to help you, but our techies will need a few more details first.
What is your Operating System?
When did this start happening, and did it follow any other significant event, like loading new software?
Have you recently added or modified any hardware, or updated any drivers?
Do you hear the single 'beep' to tell you that your system has started correctly? If not, do you hear multiple beeps or none at all?
Does your system run correctly once the monitor has started?
Edit: Just seen your answer to Clive's question, so you can probably ignore 2 and 3 of those.