Drain Block?

Buy a bag of washing soda and dump that down with lots of boiling water as a first try.

I have found that the only thing that works is caustic soda solution. Pour a couple of litres down the drain and leave overnight without letting any waste water go down that drain to avoid diluting the caustic soda and to give it chance to dissolve the gunge. Then run some hot water down the drain and apply alternating suction/pressure on the bath plug hole using a drain unblocking pump or sink plunger, whilst covering the overflow to avoid loosing suction/pressure. You might have to repeat this process several times if you have got a really bad blockage. If you go ahead with this, be very careful using caustic soda as it is very nasty stuff if it comes into contact with skin or eyes, or if you inhale the fumes.

You haven't flushed an over rim bowl cleaner down?

No that's not it...

Certainly one of the first things I'd think about. Any toddlers in the house? They are prone to flushing all sorts down toilets. Many menstruating females? Santitary products are a number one cause of blockages IME.

As someone has said - caustic soda solution. I'd echo the safety warning.