Our 23 year old tumble dryer (!) stopped heating the air, although the
rest of it worked.

So, I dismantled it, couldn't find what was wrong, cleaned it, and put
it back together. About 6 hours altogether. It now works perfectly.

Please, please reassure me that disconnecting it from the electricity
would not have reset it without all the dismantling?

Miele T430, by the way.