C64 not powering on - red led light not on

Were you using the original power supply? If so, they are prone due to age and heat to fail and put out too high of voltage, especially if it's one of the ones that is real heavy, which indicates the entire internals were covered in an epoxy that held in the heat. If it's failed and put out too high of voltage then chances are a bunch of internal chips have been burned out. That's why people usually use a new aftermarket power supply, or plug a over voltage failsafe circuit in between the power supply and the C64.

I would do two things: Buy an aftermarket power supply and also one of the diagnostic cartridges. Get the version that has a switch so you can run two different diagnostic programs. These will often take over control of a broken C64 and show you what components are bad. If the cartridge can't manage to even get the screen to come on I believe the cartridge will flash a diagnostic code.

By far the most common failure of a C64 is the power supply, so chances are it's failed. The only question is did it burn out the chips.

I'd also recommend you ask your question on the Lemon64 forum because there's a better chance people with the right knowledge will have answers for you. You also have a good chance on there to find somebody willing to repair it for you.

In the last 10 minutes I’ve realised that the email that contains my password reset for Lemon64 also contains my username! Doh!

So if I don’t get anywhere with the new power supply (just ordered one), I will give them a shout.