Hello, your post does not have enough information to make a useful diagnostic of what happened.
The most probable thing would be something came loose when you put the computer back together. But even that cannot explain why the system is so slow to start working again.
Allow me to make an observation. I've been working as a electronic technician for something like over 40 years or more. Actually if you include my earlier years as a radio amateur it's more like 50 or 60 years. So I think I know a thing or two about the nature of electronic devices.
The desktop computers and laptops are now have are among the most sophisticated letter Tronic devices that have ever been made. These devices compare to the type of systems used by the military, the hospitals, the government and big business people. The desktop computer is in effect a real computer that is been reduced down to the size of the desktop. If you are to count all the electronic junctions inside the thing, including all the semi conductor junctions, you get a very very large number that goes beyond human comprehension. A failure of any one of those parts can potentially cripple the computer in a way that is hard to predict.
At the present time the most effective way to diagnose a desktop computer is five-part substitution. That is just as quick and effective as using a very expensive logic analyzer to see what's going wrong with computer. When the computer just will not turn on and function, is not really very much logic you can analyze anyway.
Part substitution means replacing memory chips, the CPU, the hard drives, SSD and even the power supply. In a few rare cases even the keyboard might cause the computer to fail.
Some problems are very esoteric and are not easily understood. When that happens the usual remedy is just to replace the motherboard and not even try to figure out what went wrong with the motherboard. But some people do keep playing with the motherboard and delay find out what went wrong with that. But that is a very hard job.
My best suggestion is just to get another computer that has similar parts and then your old computer can be used as a source of replacement parts for a new one. That a is about as best you can do to try and recover some of your investment.
However, if you expect others on the form here to give you some more help, you'll have to be specific about exactly which computer you are talking about and exactly what parts and components are used in that computer and how it was put together or whether not he was a homemade or was a factory purchase computer. It was from a factory, you need to go to their website and see if you can find any information about that particular model that reveal some sort of weakness or might've been in that model. But the fact you made computers seldom have a warranty going past one year, so don't expect them to give you any help unless you had paid for a service plan.
And the next time you buy a computer you might consider getting an extended service plan if you are not comfortable with trying to diagnose computer problems yourself.
I believe this is the best honest answer I can give you.
