Starting Problems

My home computer is 8 years old and has been such a star, but is now refusing to start after being switched off every night. It started again after removing plug etc the first time this happened, but not the second. I got a rebuild to replace it ....but now it has started again. It works perfectly when it starts and is still going strong now after 10 hours. I have read that if the fan is not working at the back it can overheat but there are conflicting views on this and it doesn't seem to be warm. Could it be the fan, a faulty on switch, the battery needing replacing or what else Any expertise would be welcome.

The lack of info you've included suggests it would be a good idea to take it to a computer workshop. At 8 years old it should be on Windows 10 but is unlikely to meet the strict hardware requirements for updating to Windows 11. Also, a PC needs to be shut down through Windows before being physically shut off at a switch. Unless you are running a business leaving it on all day is unnecessary.
You've had a decent run with the Desktop or laptop so it'd worth buying a new one running Windows 11. (Check first if it works OK from a different wall socket.)

If you want to keep it for another year and one half before windows 10 expires you really need to provide more info on the make, model and everything about it you can think might be involved or just maybe google startup problems.