Look for the overflow tube. If the water level is at the top of the overflow tube, then you should adjust the tank float lower to reduce the water level. If the float is properly adjusted and the valve still is on, you may have a problem with the tank valve.
If the toilet does not flush the bowl completely
You may need to increase the amount of water used per flush. This can be adjusted by raising the tank float.
If you want to conserve water
To conserve water, you can reduce the amount of water used per flush by adjusting the tank float down. Reduce it as far as you can while still achieving satisfactory results in clearing the bowl.
How to adjust the tank float
There are several styles of floats. In older toilets, the float hangs at the end of an armature, along metal or plastic rod. Where that arm connects to the water valve there is usually a screw. Adjusting that screw will change the level at which the water valve shuts off. If there is no screw or it is already at its maximum, you may be able to bend the metal arm, but you should remove it from the valve or risk damaging the fill valve.
In newer toilets the float is often a collar around the fill valve and rides up and down vertically. The float typically has a spring clip connecting it to a metal rod which connects to the valve shut off control. Squeeze the spring clip to slide the float up or down the metal rod.
Manufacturers have several variations on the fill valve mechanism, but they all include a method for adjusting the water level. Close inspection should make it clear how to adjust your particular fill valve.