I reckon the record for the longest downtime for a computer must belong to the difference engine. From about the middle of the 19th century until the Science Museum re-built it, that’s about 150 years of downtime. My own record is a bit more modest, but more real since it is physically the same machine.
This is my Commodore Amiga, last powered on in the 1990s. Complete with 500MB GVP hard drive and Naksha mouse. As you can see it still works (although it took a few goes to get it to boot) and is rather noisy. I can’t remember how to use it, although it has a version of emacs, tex, some letters and a dial up modem connection to Demon internet and a few games I can’t remember how to play. It’s main use was Sensible Soccer so I will need to find the disks for that!
Incidentally, to close off the last post, my VPN tunnel worked!