This problem is troublesome, but not uncommon. It has a number of possible causes and fixes.
The first place to look is the water supply to the tank. If it has been inadvertently turned off, then obviously the tank will not fill. This is akin to the troubleshooting section of your old VCR manual, where it tells you to make sure the power cord is plugged in. It may sound silly, but it never hurts to check the most obvious and easily fixed of the possible causes first.
Another possibility that is very easy to fix is that the ballcock inside the tank has jammed at the top. Inside your toilet tank is a rubberized bulb connected to an arm. This bulb floats in water and controls the intake valve on the toilet tank. So, when the water level reaches a certain depth, the ballcock floats and closes the valve. However, if the ballcock is jammed in an upward position, it would keep the valve closed even if the tank is empty.
In this case, it may just be a matter of pushing the ballcock past the jam and then making sure it moves up and down freely. That alone may fix it.
A third common cause of the dry toilet problem is a clogged intake valve beneath the toilet. These valves can become clogged and then not allow water through, especially in systems with hard water or particularly old pipes.
If this is the problem – or if it is anything besides one of the two simple fixes described above – the best solution is to call a plumber. A professional can correctly identify the source of the problem and perform the necessary repairs. It’s likely not going to be a huge repair, but it’s best to entrust the job to a pro.