If the tank of your toilet isn't filling properly with water, then you won't be able to flush the toilet. There are a few reasons why the tank isn't holding the amount of water that it should. In most cases, although not all, the problem can be repaired without the need for a new toilet.
1. Float Issues
The float is the bubble type of device that determines the level of water in the tank. When the water level pushes the float to its highest setting, water stops flowing into the toilet tank.
If the float is either broken or adjusted improperly, it won't float up to the desired setting. This means the tank will stop filling prematurely, long before there is enough water to properly flush the toilet. You must either adjust the float setting or replace the float with one that functions properly.
2. Blocked Inlet
Water enters the toilet tank via the inlet valve. This water slowly fills up the tank after a flush until the float reaches the pre-set location and shuts the inlet valve. If something is wrong with the valve, then water won't flow through as it should. Issues can be a slow fill or it can be a complete lack of water.
Inlets are most often blocked by hard water deposits. A plumber can clean it out, but it will be an ongoing issue as more hard water deposits build up in the pipe again. It may be worth it to install a water softener to solve the hard water issues in your home.
3. Tank Leak
The worst issue to run into is a tank leak, as these are not repairable as a general rule. This is especially true if there is a crack or hole somewhere in the tank or where the tank attaches to the toilet. Although you can patch the crack or hole, the repair is very temporary at best. The best course of action is to replace the entire toilet.
Leaks can also happen between the tank and bowl. If the flapper at the bottom of the tank isn't sealing correctly, then water flows from the tank into the bowl between flushes. If everything is working properly, the tank will constantly refill so the toilet will always sound like it is "running," but if something is wrong with the inlet or float then you may end up with an empty tank. Replace the flapper to solve the issue.
Contact a plumber if you are having issues with your toilet.