A toilet is designed so that when the chain is pulled, water fills the bowl and flushes away the waste. A low level of water in the toilet bowl can lead to the waste not being fully flushed away. While some toilets are designed to hold smaller amounts of water while still being efficient, a low water level in a standard toilet is a cause for concern and should be diagnosed and fixed as soon as possible.
If you are unsure if the water in the toilet bowl is lower than it was before, there is a test that you can perform. Pour a few buckets of water into the toilet slowly, watching where the water level settles. If you pour slowly enough, the water settles to the level that the toilet bowl is meant to hold. Check the toilet bowl later on and after you flush to see where the water level ends up in relation to the point that you observed earlier.
The refill valve in the toilet tank may be knocked out of alignment with the overflow pipe. The refill valve should be securely clamped to the overflow pipe; if it isn't, the toilet bowl will not fill. A toilet vent may be clogged by debris, like a bird's nest, or it might be clogged with frost. The float ball may be riding too low in the tank or the handle of the toilet may be worn out, causing the water to be blocked before it can fill the bowl.
Depending on what the problem is, there are many different solutions that can fix the problem. You may need to replace the refill valve or secure it to the overflow pipe. Similarly, you can bend the float ball's rod to help it ride higher in the tank, allowing more water in. Replace the toilet handle if it is getting loose. Venting a toilet requires using a plumber's snake to clear the vent; if the toilet vents on the roof, you may need to hire a professional plumber.
Before you start any kind of toilet repair, cover the floor with a dust sheet or with newspaper to prevent the floor from becoming slick if water sloshes. Similarly, turn off the water supply valve underneath the toilet tank or turn off the main water line to prevent sudden overflows as you work.