So, it fills with water, but doesn't go down (or goes down very slowly)?
How many small children do you have in your house? Chances are good that something has been flushed down the stool that's blocking the trap.
You'll have to try and retrieve it. If you're lucky, you can reach it from the bowl. If not, you may need to take the toilet up and try and get at it from the underside.
If you do not any plumbing experience, it might be best to call a qualified plumber.
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