When you turn it on, does it start booting or there is no signal on screen? Are there any strange noises coming from the PC?
It is hard to say what is going on, but I would start by trying with another power supply unit. If a PC cannot turn on properly, in most of the cases the issue is PSU related. Seek assistance from a technician, because they usually keep lots of spare parts and may be able to try with another PSU.