I think in this case, it may be more of a hardware problem than a software one, so there is not a lot you can do on your own. I think it's better to give it to a technician
It's likely a BIOS or UEFI password prompt, and this is a security feature designed to prevent unauthorized access to your computer's basic settings and functions. If that is the case and you don't know the password, you may have to reset the BIOS/UEFI settings. You can do that by removing the CMOS battery
To see if the monitor is the problem, you can try connecting it to another one. If it works on another monitor, then the monitor may have some problem with the connection. Of course, you can also check the cable connection as well. If the problem persists, you can update the graphics drivers or just call a specialist
I mean, can you give more details? There are many reasons why your PC won't start, but you have to be more specific. Maybe you just have to clean it, or maybe the CPU is done for, or any other thing, but give some more details
