You can try to "unstick it" by:
1) remove the screen over the inlet port
2) look at the valve and spring inside the port
3) push on the valve and see if it will reseat
4) if it is broken you will need to get a new port
5) A workaround to keep you camping is to buy a one way check valve (called a vacuum breaker) and put it on your city water inlet till you can get yours fixed. You may need to remove the screen washer to install the valve; just remember to put it on the end of the check valve if you do hook up city water.
I carry a two spares as they are cheap and many campgrounds do not have them on their faucets (and they should).
Wilkins BFP- 9 3/4" Brass Hose Connection Vacuum Breaker at PlumberSurplus.com
PS - your city water inlet check valve typically breaks when who ever winterized your camper forgot to add RV antifreeze to (or blow out) the city water inlet. Water trapped in the valve will freeze and sometimes break the valve or spring.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW