sold a bike two years ago never gave it a another thought until eighteen months later when I noticed I had not cancelled the insurance due to auto renewal what are the problems with this happening I know I should have checked my renewal but at the time I was in hospital getting a new kidney fitted so not top of my list at the time
First thing to do is cancel it. All you can then do is hope no one has been riding it around on your insurance although if anything has happened it will be null and void for the rider anyway. If you are paying monthly and it's nearly up just let it run out and then cancel as normally you have to pay the full premium then receive what's left after cancellation fees. I don't expect you can do anything re getting the money back as it is down to you to keep these things up to date and you should have got a reminder. I hate these automatic renewals although it can be a help if you keep your vehicles for some time and forget to renew. Anyone can check on the DVLA site for tax/MOT and askmid https://ownvehicle.askmid.com/ tells you if it is insured.
it ran for eighteen months after I sold it with the new owner using it every day, as I said it was not top of my list at the time would I have been held to account if it had been involved in a accident its all cancelled now done so when I changed insures
I think the new owner would have been charged with having no insurance as it was not in his name. If he took out his own insurance that's fine and doesn't really affect whether you had not cancelled yours.