It depends upon how much time you have to work with the tank now. If time is no factor, fill it with water and let it soak with a bottle of drain opener enzyme eaters. Even Rid-X, if given enough time, will break down the mess. But it takes time for this work work, and then lots of flushing, and maybe even repeat treatments with an enzyme eater.
Click here.Any one of these will work (if given time).
If time is not an option, then some of the suggestions above are probably your best choices.
You can also open the outside valve and shove a garden hose up the pipes, turn the water on and try to break the "stuff" up. It's a dirty job.... for sure.
Another option is to shove a garden hose down the toilet. But one of these is much easier:
Click here.You attach a garden hose to this then put it down the toilet, turn on the valve and let it spray, moving it around. Make sure the outside valve is open as things begin to break up, you want it to flush out.
One more thing, simply look inside the toilet and push the peddle open and look down the hole. Make sure nothing is clogged in the neck of the toilet. If you can't see the tank below "stuff" if clogged up there (like gobs of toilet paper), simply use a stick and push it on down.
Good luck!