Clothes dryers need two things to work, heat and air flow. The absence of either one of these will prevent the dryer from working correctly. Different brands have different weaknesses, but as a general guide the following could be at fault -
When a dryer looses heat it can be caused by several faults within the machine. Dryers are essentially bar heaters with flammable clothes inside them. Therefor every aspect of the dryer is in series so if one component fails heat will be removed to avoid a fire.
The way a dryer is setup is power comes from the switch or timer to the fan and heater. Firstly it must pass through a set of safety thermostats which remove power to the heater when an over temperature situation is detected. These can fail, causing heat to be lost. Some of these thermostats are located around the machine as small buttons. By pressing them in they reset the situation and enable drying to resume. REMEMBER: something has caused the thermostat to trip so simply resetting may cause the fault to return. Always check the filter is clean, the intakes and exhausts are clear and you can feel airflow exiting the machine.
No air flow
Airflow is critical in a dryer. Any interruption to the airflow will cause the machine to overheat and eventually shutdown. A clear intake or source of air is required. Make sure there are no objects that have fallen over the inlet or exhaust. I have seen many missing socks inside intakes.
Within the machine there will be a fan belt . You may not be able to see if the belt is ok but a indication is if you can feel air being sucked in, or blown out.
NOTE: just because the drum is spinning doesn’t mean the fan is. Many dryers these days use separate belts for fan and drum.
Always keeps filters clean. Some driers have more than one filter so be aware there maybe more filters. Condenser dryers always have filters and condenser cartridges. These need to be kept clean and washed out on a regular basis. (refer to user manual)
If your dryer has a duct leading outside a trick we occasionally come across is that something has been lent against the vent outside. Check this also.