typically the hot suface ignitor wears out, hence no ignition of the gas and no heat! They are very brittle and very fragile. They range in price from $12-28.00. If you add a $75.00 - $125.00 service call on top of that your coming closer to buying a new dryer!!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix tumble dryers.