Pc just "lights" but doesnt turn on after swapping psu n pc case

I swapped psu n pc case, both brand new, now pc doesnt wanna turn on, it just lights for a second then immediately turns off, and also there was one situation it didnt turn off after lightning but was just lightning like every 3 seconds and the cpu and dram was lighting up at mobo, please help me P.S. its the same with old psu that was working good