I appreciate the point brought out by mikeh. If there is a sudden hearing loss, this can be a serious medical condition. If you sense just a minor blocked feeling like you would if you had an earplug in, it may be wax or as others have mentioned, a eustachian tube dysfunction which will create pressure on the eardrum, or even middle ear fluid (fluid collecting behind the eardrum), like is common in young children.
There is, however, a somewhat rare condition called a sudden sensorineural hearing loss, where the hearing can go completely in an instant. We see one or two people with this problem every month or so who report this and will often see a "dead ear" or profound hearing loss in one ear. Usually this occurs unilaterally, but I have occasionally seen this in both ears, which is tragic. The key here is to seek medical attention ASAP. Most Ear, Nose, and Throat physicians will prescribe a steroid for 10 days or so, which will often help restore the hearing -- at least partially, however this does not happen in all cases. There is a lot still not understood about the causes, but many are believed to be viral, while others are an autoimmune inner ear disease, where the bodies immune system attacks the inner ear.
Sorry to provide so smuch information on this, I just wanted to warn that if you feel the hearing is completely gone in one ear it is an urgent problem. Be persistent and insist on seeing an ENT as soon as possible. Most ENTs consider this a serious emergency and if you mention sudden hearing loss will get you in same day if at all possible. Studies show that the steroids have to be started within 2-3 days or efficacy drops dramatically. Often ER physicians and primary care physicians do not understand this problem well and don't understand the urgency, so if anyone ever experiences this or knows someone who does, please be persistent as you seek medical attention and get to an ENT as soon as you can.