Why has my wifi changed from belkin to ‘should have got a password’?

It sounds like your Wifi has been hacked by a neighbor. Your best suggestion at this point is to factory reset it and configure it for WPK or at the very least WEP.
You should be able to find instructions on the belkin website.

This also means that more than likely someone has been using your wifi for some time now and finally decided to make changes.