Why did my coverage expire and how do I get new coverage? When I try to it says my phone is not eligible.

Did you have Apple Care+? Or did you only have the 1 year warranty?

Apple Care+ if bought as a one time payment expires 2 years after purchase. It is not forever. You can then extend it as a monthly payment indefinitely after it expires if you want to. click here-> Extend your AppleCare+ coverage - Apple Support

If you just had the 1 year warranty, then that only lasts 1 year with no option of extending it.

If you had the monthly payment version of Apple Care+ then that lasts for as long as you pay for it.

The warranty is one year. If you purchased AppleCare+ at the time of purchase, it was good for a total of 2 years from the date of purchase. Once it expires, you can't renew it.