Check the cycle selector control to make sure a heating cycle has been selected. The selector should be in an "Off" position when not in use, but it should be turned to a drying cycle when in use. On some models, make sure you are not accidentally running the dryer on the "Air" cycle, which does not use heat.
Check the gas supply valve to make sure it is open if your dryer is a gas dryer. If it is not open, just lift the valve open.
Look at your house's main breaker box to determine whether a circuit breaker is tripped. The electric dryer runs on 220-240 volts and has a double breaker. If the breaker for the heating element is tripped, the drum may tumble the clothes without heat. Turn the dryer's breakers to off, then back on; test whether the dryer will produce heat.
Consult a technician if troubleshooting does not resolve the problem.