If your dryer isn't producing any heat but it's tumbling, there's a couple of issues that could cause this problem. However, look for the obvious first by checking your dryer's settings. Be sure that you have the appropriate settings for the clothes that you are drying. The fabric temperature and timer selections are all key to the performance of your dryer.
Many dryers have a safety feature called a thermal fuse which protects the dryer from over-heating. The thermal fuse is generally in a white plastic housing and it's located in the exhaust duct in the back panel of the dryer.
The thermal fuse will "trip" and prevent your dryer from working if it gets too hot. There's not a way to reset the fuse, so if this is your problem you'll need to replace the thermal fuse with a new one.