Have you tried unplugging the monitor cable from the graphics card at the back of the PC and plugging it back in again, checking it for bent pins in the connector?
Also try unplugging both the PC and the monitor from the mains power for a minute or two and then plugging them both back in and switching them on again.
Was the PC working OK the last time that you used it? Has any work been done on the PC (e.g. hardware changes inside the base unit) just before the problem started?
Are you able to try a different monitor or try your monitor on a different PC? You might find that the PC is fine but the tube in the monitor has blown.
