Water has quit running thru my humidifier.
Novice
Check the humidity
The humidifier should only turn on if the humidity is below a set level.  Make sure the humidity is actually below this level.
Clean the inlet
Check the inlet (where the water comes in), and make sure it's not all gunked up.
Check the valve
If the humidifier supply line connects to the water supply via a saddle valve, close and reopen the valve (and consider replacing the valve, which will require shutting off the water since the valve pokes a hole in the pipe).
Set the humidistat
If a humidistat is used, make sure it's set properly.
Intermediate
Check for water at the solenoid
Close the valve supplying the humidifier, and remove the supply tube from the supply side of the solenoid. Open the valve just a bit, and make sure water is coming from the tube.
TIP: Wrap a rag/towel around the open end of the hose, and hold it tight when opening the valve.  The rag/towel will prevent the water from spraying out the end of the tube. Also, don't forget to hold the rag/towel wrapped tube over a bucket.
Check for power at the humidistat
Open up the humidistat, and use a multimeter/voltmeter to make sure the humidistat has power (most likely 24 volts).
NOTE: Depending on the installation, the humidistat may only have power when the furnace is running.
Check for power at the solenoid
With the humidistat open, and the furnace running. Set the humidistat to the highest setting, and check to see if you have voltage going to the solenoid.