When I start it, won't come up on the monitor

If it EVER works, then the problem is not a failed part, but a weak part. The most likely component type to give you such problems are capacitors. But very few repair techs know how to find such problems, and even fewer have the equipment needed to find the bad one(s) in-circuit. The failure mode is high ESR (ESR means Equivalent Series Resistance). The lower the ESR, the better. Capacitors with high ESR cannot stabilize circuits which demand high pulse currents, such as in switch mode power supplies.

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this can be a sign that onboard video is not working please try to add a separate graphic card

check for any blown capacitors at the power supply board.

What to you mean by Monitor won't go on? Are you saying the power on the monitor will not turn on? Are you saying the Monitor is on, but there is nothing showing on the screen? Also it would help if we know what kind of computer and what operating system.

What to you mean by Monitor won't go on? Are you saying the power on the monitor will not turn on? Are you saying the Monitor is on, but there is nothing showing on the screen? Also it would help if we know what kind of computer and what operating system.

First, it could be out of adjustment--turn the brightness and contrast controls up and down to see if it makes a difference.
You could have a bad video card in the computer. Do you have another monitor to try to see if the actual video output is good? This is the easiest way of troubleshooting.
When you start the computer, can you see any white printing on the screen indicating it is booting? Does the problem start once Windows starts? If so, try starting in safe mode. If you can do this, then I'd suspect video driver problems.

VGA is an analog connection, so you should connect it to your monitor BEFORE starting up the PC. Otherwise it won't show anything

May be the monitor needs to be serviced.
Most surely there may be capacitor leakage which results in such problem.

won,t start in first try it start after 15 to 20 try,i have dell monitor p1230