When combined with the monitor, my CPU is not starting . What do I do since when it is alone it was doing well?

Please clarify what you mean. Your statement is unclear. Also provide details about your computer (specifications).

Based on your unclear description, the best I can tell is that your computer does not boot when your monitor is connected. Are you sure that it is still booting, but that you have no video? If the latter is the case, then I would try a different monitor and a different monitor cable. Or, try the monitor on a different computer.

But as I mentioned before, more details are a must.