nullaii

My bro has a prebuilt pc. He decided to build a new pc, where he has bought all parts with my help except from the gpu and ram which he would be using from his pre built. I helped him over video call as he built the pc. Fast forward, pc built, plugged in and everything. All fans spinning, but no output. Monitor and everything works. Puts ram and gpu back into old PC and it all works again. Tried repeating gpu n ram, made sure all cables connected properly. No luck. Any idea?