Hardware can go bad and connections can loosen over time. I've had drives disappear only from the cable coming loose. Unplugging and plugging back in solved the issues. My current desktop is only report 48GB RAM when there's 64 installed. Undoubtedly at least one of the 8 DIMMs has become unseated or gone bad, but I've been too lazy to troubleshoot because I don't use that much currently.

I have a laptop that was stable for 18 months, then started crashing with increasing frequency. Booted in to memcheck and the memory was faulty. Replaced it and the crashes went away.

Not saying it's definitely the cause, but strongly suggest you check if you haven't yet. It doesn't take long.

My system blew a RAM chip after 6 years. Changed the RAM and it's all good again.

We sometimes fry CPUs, memory controllers or RAM at office. Even a well cooled component has a lifetime and eventually gives up.