What should I do if I missed the renewal for my retirement visa in Thailand?

Presumably you have Thai bank account? If you have 0 baht in it then you dont exactly have to start again. Apply for non O in the country you are in and enter Thailand then get 12 month extension at the appropriate time. OR Enter Thailand visa exempt for 60 days and get 30 day extension if you want. Then get 90 day non o in country at the appropriate time Followed by 12 month extension. If you dont have a Thai bank account with 0 baht you should get a non O out of Thailand and open one and season the 0 baht before getting 12 month extension. Use the options to plan your future renewal date to possibly not coincide with when you are not in Thailand. I did this to move my renewal date away from the summer months when I am in the UK.

Now you're just a tourist

Sadly yes, back to square one and start over

Yes, obviously.

Your only option is start with a new 90 day non-O visa

You had a retirement visa, now you don't. Got to start again.