Added to what Jay has said there is a website called HomeTrends or www.hometrends.com that provides home owners with simple equipment and other neat and useful gadgets one of them being a dryer vent brush. This tool is under $20 and believe you me when i tell you will be the best $20 you ever spent. It's a hard bristle 4" brush with a 10'lead. I do this for a living so i know. First find the outside vent, if it has a cover or cage remove it. Then turn on the dryer and and snake the vent from the outside this will loosen debris and the dryer running will force it out. Next go back in side move the dryer and and disconnect the hose. Snake the vent from this way most dryer vents are not over 20' this is why you need to do it both ways. When the loose debris comes back with the brush clean that also snake inside the dryer to loosen lint. Next reconnect the dryer and turn it on look outside to see if debris is coming out it should have if not there is a good chance that there is a clog. If so do what Jay says and climb under the house if that option is available. Hope this info helps out you or anyone else that reads this post..