I'd start with replacing the small battery (a CR2032 or something like that) on the motherboard.
So a year ago my Dell desktop tower was working normally; then, after being off and unplugged for about three weeks, I tried to start it up, and the boot sequence wasn't loading properly. I tried to do it from UEFI, then Classic, and back, to no avail. I even tried downloading Windows 10 onto a flash drive, and tried reloading it, and came up empty there. I'm thinking of taking it to a repair place to get looked at. Is there any way I can fix this on my own? Thanks!