I have an up all in one pc.. just recently it started shutting down about 5 seconds after powering on.. what causes this?

Yes, it could be overheating or it could be failing the Power-On Self-Test for another reason such as a bad CPU. Some systems are programmed to shut off immediately if it doesn't detect a CPU fan. The Mainboard could be bad, short-circuit, power supply problem, loose power cable, etc.

All-In-One PC's are notoriously bad for breaking down due to overheating problems, especially the budget models which aren't built well to begin with.

If you aren't willing to open it up and clean it, then you'll have to take it to a repair shop.

Usually if a PC dies after a few seconds and doesn’t give an error, I means it is overheating. Clean all the vents and try to use canned air to clean the fan. This normally fixes the issue

it needs a technician's attention.

1. Overheating. Open it up and clean out the insides, The heat sink over the CPU.

2. Faulty Power Supply.