You wouldn’t think that memory that was working perfectly for two years would up and fail causing a computer to stop booting. And you would be wrong. I had just such an event happen to me.
It manifested itself as randomly crashing software initially. This meant the problem could be anything from the processor to the memory to the hard drive to the software on the system being messed up. Eventually it got to the point that I could do nothing with the computer so I decided to troubleshoot it. I made sure everything was assembled correctly and found nothing wrong there. I checked for overheating and found no heat problems. Then I swapped the memory with that from another computer. And lo, the other computer started crashing and the first one became stable.
So now I’m $100 poorer but have shiny new memory in my computer.
I hate when computer hardware fails.