Panicbutton's advice is OK if it is just a small blockage, i.e an unflushing lavvy.
However, it the problem is in the sewer pipe, you must call Dyno-Rod. The cost at that time (5 years ago) was certainly no more than �50, indeed much less. It was affordable anyway. They had to get into outside drains and poke rods through the pipes. They did a good job, and it had to be done.
I used to be a cleaner in a club ( at weekends the worst time to ever work ! ) If the loo was blocked we put our gloves on and a plastic carrier bag over that , and had a delve around , next step was the plunger , if no result then the call out service , most of the time it was a towel of some description , but Dyno Rod are not cheap but maybe you can get it back on your buildings insurance ?