Why does my camera automatically power off after a few seconds?

Not an expert on camera electronics but may suggest a few simple things to check, based on your diy skills. If disassembly doesn't scare you then you may be able to verify power is adequate (battery or adapter) and reaching input connections. Verifying actual power is reaching main power connections is important to separate a power issue from camera electronics. I tend to presume power first before considering electronics failure. Of course, random electronics failure do occur whether inflicted from external influences or internal. Using a multimeter is crucial here for diy diagnosing.