Welcome (back?) to the Amiga, I hope you get this sorted soon.
When it powers on, does the floppy drive click every couple of seconds (assuming it has one, and not replaced by a Gotek)?
If you list the upgrades here, it might also help identify the cause.
There are loads of active Amiga groups in the UK so if we can't fix this remotely, you might find a local group to help out. Depending on where you bought this, there may be some buyer protection too.