This could be due to use of too much toilet paper. It could also be due to other materials being dropped into the toilet such as combs, washcloths, diapers,
socks, brushes, etc.

First try a "plumbers friend" or plunger. A plunger for the toilet is different than one for a lavatory, although there is a type which can be used for both. If this does not clear the stoppage use a plumber's snake made for a toilet. This differs from a lavatory snake because it has a section of tubing which can be pushed into the liquid in the toilet.

The outlet is the large opening usually toward the back. With the pipe handle inserted in the outlet, gradually turn as you insert the spring into the drain. The spring has a hook on the tip so when you feel it catch on something continue to turn the handle slowly and pull the stoppage out.

If the plunger or the snake do not remove the stoppage, call a plumber.