Hi!
Could be a lot of things to be honest!
First things first, how is the power supply? Is the LED light burning?
Did you tried another PSU? Would try this first before proceeding.
If the PSU is good, it's most likely something wrong on the motherboard.
You can disassemble it and take a look at it. Otherwise you need to run some tests.
If you need any guide, please let me know!