Why is it powering on then off

So I was playing my game and I had my Xbox on top of the mantel which it had fell, after picking it up and pluging it in it would turn on then 4 seconds later it would shut off. I had opened the console for the first time in 3 years it was really dusty I clean it and assume that was the problem so i connected it back it the light was on the fan was spinning but when i unplugged it and put everything back and plugged it back in the same thing started to happen. I took it to get it checked but they couldn't find the issue with it. I just need a 2nd opinion