It's not uncommon to experience problems with the tap on a beer pump from time to time. If you notice the beer is pouring at a slower speed than usual or not coming out at all when the tap is open, it may indicate problems with the line. There area few relatively easy steps you can take to get the beer flowing again.
Check to make sure the keg is not empty. This is one reason why beer won't come out of the tap.
Inspect the regulator gauge on the outside of the keg where the beer is stored. This controls the carbon dioxide pressure. The settings may be off, causing too much air and not enough pressure into the line to move the beer from the keg to the tap and eventually into a glass. According to probrewer.com, the optimal setting is between 13 and 18 psi.
Disconnect the lines from the tap to the keg and clean with soapy water. There may be a clog in the lines preventing beer from coming out of the tap. You may need a wrench to loosen the brackets holding the both ends of the line in place. Make sure you rinse the line thoroughly before reconnecting to avoid soap from getting into the beer.
Check for any kinks in the line. Any kink or obstruction will prevent the beer stored in the keg from making its way to the tap.