You will easily know if your dryer is problematic and has to be changed or fixed. First, it takes a much longer time for clothes to dry. Compare the time it takes for your clothes to dry before and now that a problem has existed. If the heat is trapped, it easily dries up the clothes. If there is blockage, it will work harder. This could even lead to fire.
You should also touch the top part of the dryer while it is running. If it is too hot, this means that heat is not exhausted of the dryer properly. This may also lead to a fire. As soon as you notice the problem, stop using the dryer.
Call an expert to come over and fix the dryer. The vent might need cleaning or some other parts might need replacement. Either way, you need a professional to do the job.