Welcome to the club. Unfortunately, you are likely in for an extremely nasty and unpleasant job.
Do the following:
1. Wait another day. Often the paper will dissolve. Try flushing
2. If that doesn't work
, then take apart the pump
and see if the macerator is clogged up. If you're lucky, that all it is. Clean it out, wash it all thoroughly and reassemble.
3. If that doesn't do it, then you have to chase the pipes. Take the pipe off the macerator and off the sea cock and see what you can find. Sometimes you can snake through a coat hanger or something like that and get the clog. Often, you can't. You might remove the anti-siphon loop to get additional access through there.
4. If that fails -- and it often does -- the classical solution is just to replace the pipes -- the need regular replacement anyway so here's your chance. Or if they're new, take them off and snake them properly off the boat
Here's a trick I discovered: If you can get to a holding tank
pumpout station, try to make a good fit on the waste pipe you have taken off the sea cock with the pumpout nozzle. Make sure the waste pipe is disconnected from the macerator, but connected at the antisiphon loop. Suck away. For me, it worked like magic.
P.S. when you have the waste pipe off the sea cock, snake a coat hanger through the sea cock as well just to be sure that your clog is not there.
P.P.S. all of the above is for direct discharge. If you discharge through a holding tank
the procedure will be slightly different.