Take out the memory fitted to the motherboard.
If it produces an error code, or an audible beeping it confirms the mother board and bios are still working.
If it produces no error code or audible bios beep code for no memory fitted it is likely the bios is corrupted or the board is damaged in some way.
It may be as simple as the fact that the cpu you replaced and put in the cpu socket of the motherboard is currently not detected because the Bios firmware of the motherboard needs to be updated first of all to support the cpu you are now using.
It will result in a black screen and the system not powering up if the case Andre.
Gte the brand name of the motherboard and it`s model number that can be found by looking at the motherboard where it will be silk screen printed on the motherboard it`s self.
Look it via google, and see if you can find a download and support page for the motherboard. and check what cpu models the board will accept with what revision of the bios firmware upgrade patches.
If it requires a firmware bios upgrade then follow the instructions on how to download the bios file and flash the bios firmware of the motherboard to a higher version of the bios software.
Once applied if the case the board should then post and power up right.
Or post the brand of motherboard it is the system has and it`s model number plus the exact model number of the cpu you have fitted to the cpu socket of the motherboard for help ok.