The manual for this phone can be viewed here:
On Manual Page 20 / pdf page 23, it says that if the ringer is muted for a single call, that it should work again the next time someone calls.
However. page 57 notes that the base ringer volume can be set between 0 and 6, so it's possible your ringer volume is not technically muted, but just turned all the way down.
Manual page 62/pdf 67 has a half-page list of other reasons that the ringer might not ring. (It can also be turned off, and there are different adjustments for both the phone and the base unit.
Hoing this lets you figure it out!