I assume the computer has worked in the past.
The 24 pin power cable is seated properly
The 4/8 pin CPU power cable is seated properly
The CPU is properly seated and the CPU fan is firmly attached w/ thermal grease
You have the front panel connectors plugged into the motherboard properly
The power supply is plugged into a wall socket that has power.
If all that is happening and you still get nothing, it's likely time for a new power supply. If you’re getting fans spinning but no post, it could be the GPU, CPU or motherboard. Send more symptoms.