The following procedures will help you repair your dryer whether it is gas or electric. While doing any resistance checks on the unit make sure it is unplugged as you will be exposed to lethal voltages if the unit is not dieconnected. Additionally when doing resistance checks remove one wire of the circuit, replace the wire when done. The best place to get parts for any appliance, best price and availability, is at http://www.repairclinic.com
Electric dryers are probably the most common so I will address that first. If the unit spins we know it has power. Move to the next paragraph. If it doesn't spin check the receptacle for 120 vac. If you have it. check the door switch for continuity. Remove the top to access the switch. Using a DMM, on resistance scale, depress the switch and see that it has no resistance. If this checks bad (high resistance) replace it. If good access the start switch. This may require removing the control panel from the top of the unit. While engaging the start switch check for continuity. If open, replace it.
The most likely culprit, when the unit spins but does not heat, on an electric unit, is the heating element. Once used they get brittle and any moving around can crack them. With the unit unplugged remove the back. Should only be several screws around the perimeter. Some units have an inspection plate which can be removed to access the element. Using a DMM on resistance, check the continuity of the coil. It should read several ohms if good. If it reads infinity, you need a new heating element.
If the coil checks OK you will need to check the limit switch. It is located somewhere in the vent area and shuts the unit down if it overheats. Many times these bimetal switches stick open. It looks like a hocky puck mounted to the vent. Once located remove at least one wire and check for continuity. It should have no resistance. If it has a high resistance it will need to be replaced. This should solve your problems if you have an electric dryer. Either the heating element, door switch, start switch or the limit switch will be bad.
If you have a gas dryer the resistive coil is replaced by a gas burner which is a little more difficult, and dangerous, to troubleshoot. If it is less than ten years old it should have electric start. Open the inspection door on the bottom front and see that when you call for drying, (start it), the igniter sparks. If it doesn't and it obviously has power the igniter box will need to be replaced.
If the igniter works and you know that you have gas (other gas appliances work) and the gas valve on the dryer is on, then the control valve has a problem and will need replaced. This may be a job for a professional as working with gas is dangerous and you should be qualified to do the repair.
Beyond those two items there is a light sensor in case the flame doesn't start or goes out. This shuts the gas valve but would only be a factor in the repair if the units burner lights up, runs for a minute and shuts down. It is a photo cell located in the area of the burner.
There is also a tempreture sensor but this would only be a factor if the unit heats but not to operating level. It is similar to the limit switch on an electric dryer and the same troubleshooting applies.
With the above information you should be able to repair any dryer on the planet. Good luck.
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