I’ll be honest with you. Don’t bother with chemicals! Drain cleaners are always the nastiest chemicals one can by over the counter. It just creates a hazard once you bring out the big guns! Sure, sometimes a careful glug of concentrated sulfuric acid can shift that pesky bung in your pipe but if you have a big block then it’s not going to get the job done. Start with a plunger.
Your first port of call should be an ordinary plunger, much like the one depicted above. Plungers get fancier the more money you spend; some squirt water while others blast air from compressed chambers. A plunger might well get the job done. I’ve been to several scenarios where a pump of water jetted down the pipe has shifted a block in no time at all. Plungers are really just for sinks and an awful design for toilets. For all your bravery and enthusiasm in taking a plunger to a toilet, the last thing you want to be doing is splushing yourself with warm brown water and ruining the carpet. If it’s a toilet block then 9 times out of 10 you’ll need to rod the drain that drains that toilet. There should be an inspection hatch in the garden outside. Turn on some taps so that you can see what’s draining where and so you know which underground pipe to rod.
Rodding is absolutely no fun at all, although there is a certain satisfaction when the pipe unblocks and splutters up that which was choking it, it is otherwise a deeply unpleasant experience. You may wish to call a plumber who will at least get paid for the job.