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 replaced all the support rollers. Cost on this repair was 300.00.
This is a dryer motor This dirt can cause the motor to over heat 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.