My pc won't turn on after i took it out of my case for cleaning. then when i put it back in it would'nt turn on.

Dec 11, 2020
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Specs
Asus h81m-c motherboard
i5 4460 intel processor
Asus 1060 gtx graphics card
550-watt power supply corsair
16 gigs of ram
1 terabyte disk
Problem
My problem is that my pc won't turn on. this occurred to me after I switched my thermal paste. I have a quite old pc around 5-6 years. so I switched the thermal paste and put the CPU fan cooler on again. then I took all the cables and placed them back in the motherboard. then I tried to turn it on and baam the power switch was not lighting up and the pc didn't start. This made me wonder why it wouldn't start since it could start before I switched thermal paste. but it would turn off some seconds later. if you have any way of helping me please do so.
 
ok now i took everything out and took it back in. though it now has a problem where it shuts down after 10-15 seconds. is the anything I can do so it doesn't shut down?