***This is a pretty easy fix***
There are tones of things you can do before truly verifying it's a dead CPU or GPU. Easy steps (especially on this older laptop) are:
* Take out both of the RAM Dims and reseat them. This seems like a really basic tip but it the laptop was dropped even on a bed or seat it can dislodge the RAM dims. Also, if you have another RAM stick laying around, take out both of the RAM Dims out and put a good known working RAM stick in and see if it posts.
* Make sure if your CPU is unsockitable (a.k.a there is a lever to take it out) its been put back properly. This can cause the system to not boost/post properly.
* Replace the CMOS battery - This can cause the BIOS to reset and not know what the time is and it throws it off for some random reason.
* Plug it in to an external monitor - Theres always a chance while replacing the power button you damaged the screen connector or something so plug it in to a TV or Monitor just to verify that's not the issue.
* Take it to a repair shop and request a BIOS reset. On old laptop like these, there's always a chance that the BIOS just decided to go 'meh'.
* If you are really determined to get the laptop working and nothing is damaged, douse it in rubbing alcohol. This stuff is like miracle liquid for technology. It can make something that seems as dead as ever come back to life in 123. You can buy rubbing alchol on Amazon.
Alternatively, just get a new Laptop. I recommend refurbished if a new Laptop is completely out of your budget.
Hope this helps - Bezzzles.