That's an easy one... clear the blockage.

You should be asking "why is my toilet blocked up"

And have you used a plunger yet?

You have a blockage, use a plunger as smoothy suggested if that doesn'r work use an Auger, see the images below, good luck.

John

I feel every house... condo or apartment should have at least one plunger in it.

They are cheap.. easy to use, and everyone is going to need one on occaision.

If the water is rising fast, you have only a few seconds to act.

1) Take the lid off the tank and set it on the toilet bowl or the floor.
2) Push the "flapper" back down so it seals the large hole in the bottom of the tank. The "flapper" will tend to float after the lever pulls it up at the beginning of the flush.
3) Lift float to stop the fill valve and prevent more water from entering the tank. Old fashioned toilets have a big float ball on the end of a rod. Newer ones have a float cylinder wrapped around the fill valve assembly.

Having a plunger is handy to fix a plugged toilet but does nothing to stop an overflow.

Hi Dgalt...

Great job answering the question as presented!

Only other thing to add would be to turn the toilet shutoff off once you have the overflow condition stopped. If the shutoff is seized then you can tape the fill valve in the up position, or use a coat hanger to hold the fill valve up until you can unclog the john... ;)

Nice job!

Mark