Fixing a broken loo is not one of those jobs that can be put off. But before you call in the cavalry, bail out the excess water and try a couple of DIY fixes.
Put a plunger over the drain opening so it seals it completely. Plunge vigorously 10 or 12 times, then yank the plunger out with force. Repeat as many times as needed.
If you don’t have a plunger to hand, improvise by wrapping a plastic bag around a mop head.
Secure the bag to the mop handle first so that it can’t work free.
Never use a plunger straight after a chemical drain treatment in case you splash yourself with harsh chemicals.
Probe the blockage using a plumber’s snake, or auger. This is a crank-handled tool that winds a cable with a hooked spring tip through the toilet’s U-bend trap and into the drain.
Use the auger to grab the blockage and very carefully pull it back out through the trap, or to push through and break it up. If you can see the blockage, bend a hook in a wire coathanger to try to fish it out.