Just a friendly message to everyone interested saying I'm now back!
The school didn't tell me exactly what was wrong with it or how they fixed it. This is a shame, because I'd quite like to be a computer tech expert/troubleshooter on an amateur level at least. The problem was an interesting one that I'd like to fix myself. I strongly suspect that it was ultimately fixed by being turned off and on again, but in a more complicated way that involved interfering with the actual hardware. See inactivity warning to read what was actually wrong with it.
