Is it electric or gas?
If it's gas, there may be a problem with the hook-up. If it's electric it may be the heating element
When you turn it on, do the clothes tumble inside?
The number one cause of problems is the belt slipping or falling off. If the drum doesn't turn, the dryers won't heat up as a saftey feature so the clothes won't catch fire
Are you getting any warmth at all or is it still cold?
Possibly a loose connection at the element if it only gets warm
When you set the timer, is it for a certain number of minutes or a degree of dryness?
Some dryers have a sensor that measure the humidity of the exhaust air of the dryer and turn off based on how dry the clothes are. If you have it set to dry to a certain amount and you aren't getting any heat, it will just run for hours since it won't dry.
All in all, you may have to replace the dryer element which doesn't cost that much and even you can do it with a screw driver.