There are a lot of causes for this issue but the first and most likely suspect is the PSU, especially if it is a generic model. Please post the complete PSU model number. You should borrow a spare PSU from a friend and test it to your system. If you don't have that capability take you PC to a repair/service shop. If you manage to test you system with another PSU and it still doesn't work then you may have a bad motherboard.