Folks everyone forgets about their dryer till it doesn't heat.

The biggest cause is dirt. This picture is of an electric dryer about 3 years old. This not only causes the dryer not to heat right it's a fire hazard, dust and lint is highly flammable.

It can cause the element to burn out the blower motor to quite and thermal fuses to go out. leading to larger repair costs.

Get them cleaned every 2 years, if you have pets every year.

A service call to clean can cost around 90 to 150 dollars in this area more in other part of the country. depending on the brand and company. but add parts and you start to get even higher costs. A heating element runs around $75 to 100 plus labor.,so now your around $200. 

This is a more resent dryer I worked on the dirt and lint (pet hair) was even worse before I cleaned it out, the lint was inside the heating element, surprisingly the element still worked (should have burned out or caught on fire. took me an hour and half to clean and then had to return to replace all the support rollers. Cost on this repair was 300.00. 

This is a dryer motor. This dirt can cause the motor to overheat causing motor failure. A major cost in repair depending on the manufacture your looking 300 to 400 dollars. compared to a service call to clean this of around 100 to 150.00

Gas dryer burner. Leave a dryer running while you run up to the store? 

No house when you come home. This is highly Flammable.

Please turn off appliances before leaving. This includes Ovens and Washers. Why washers? If a hose breaks you could have a major flood while you're gone. If your washer is in a finished basement, you can have damaged to carpet and walls if upstairs which a lot of people are doing you could have major damage to walls floors and other areas.