The most common cause for dryer overheating is a clogged dryer vent. It is extremely important to have your dryer vent cleaned out annually at a minimum and preferably bi-annually to prevent a dangerous fire hazard. You should clean your lint trap before and after each load.
There might be lint trapped in either the lint screen or in the lint line that runs from your dryer to outside the house. Try either vacuuming the screen or line or calling a professional. You should take care of this ASAP becuase lint can catch fire pretty easy.
Try checking all of the lint traps: the one in the dryer that you clean before each use and the one outside of the house. Lint buildup causes overheating.
Your lint trap may be full. This should be cleaned at least every couple of dryer loads if not more often. This can cause your dryer to overheat, or in some instances cause a dryer fire.
Two possibilities: one is that the vent is clogged up. Undo the hose at the back and clean out the entire hose. Unscrew the back and clean out the place where the hose goes in. Another possibility is that the drum is not turning. Turn on the drier and check listen to make sure the drum is turning.