If you have a blank screen, how do you know the keyboard and mouse don't work? Did you try a different monitor? Check all monitor connections? If a different monitor also has no display, need to start testing components on the computer, first check the motherboard for any damage to the capacitors, common issue with computers actually getting power but not working. Then try a new/different working power supply, then RAM, then CPU.

If you don't have any components to test with, you'd just have to replace the PC, or at least try a new power supply and maybe the motherboard.