Probably a dead PSU. Try finding a friend and use theirs or just buy another (same/greater wattage ofc).
If the new PSU works and the old one doesn't = old PSU is dead, replace.
If the new and old PSU don't work = MoBo probably shot, highly unlikely but happened to my custom build before like around a year ago.
P.S. It's a lot cheaper building your own computer instead of having some company overcharge you.