@Toinxc,
Even grosser, you might have to put your arm in there and flush again. Won't the suction cup of the plunger go back on?
Get a bunch of towels and bunch them up around the toilet.
Open the lid and jiggle that chain thing.
Ask a neighbour.
This is all probably way too late to help you - looks like 3 hours too late.
Let us know what you did that worked.