As a long time band director I have seen many strange things happen during my stdent's trumpet lessons. Despite repeated instruction during the trumpet lesson on how to care for and maintain the instrument someone would always have a problem. This article lists the most common problems that have come up during my student's trumpet lessons and how to solve them.
Most of the time this problem is really a result of not having the trumpet's valves seated correctly in the casings. If a trumpet student removes all of the valves to clean or lubricate them he or she will often put them back in incorrectly. If you cannot get any air through the horn at all then it is a good bet that at least the first valve is seated wrong. Unscrew the top cap, pull the valve out, and make sure that the number 1 is stamped on the valve. If it says 2 or 3 then the valve is in the wrong casing. If you have the correct valves in the correct casings then check to make sure that the valve guide is locking into the slot inside the casing. If the valve is locked into the slot correctly then the valve will not be able to spin. Try spinning the valve around inside the casing until you feel it lock into place.
Most stuck valves are caused by one of three issues. First, the valve my simply be dirty and the built up gunk may be slowing the valve's action or stopping it completely. Second, one or more of the valve slides attached to the stuck valve may be bent in slightly causing the trumpet's valve to bind up when it gets down to that part of the valve casing. Lastly, any dent in the outer valve casing will make the valve get stuck. Even the slightest dent (such as one caused by hitting a music stand) can make the valve stop working.
One other thing that can cause stuck trumpet valves is if the button's stem is bent to one side. As the button is pressed a bent stem will rub against the side of the hole it is traveling through. There should always be the same space around the valve stem no matter how low the button is pressed. In most cases gentle pressure can bend a bent valve stem back into place.
This is an easy repair, but most people do not have the proper size cork or adhesive to fix it at home. For best results take the trumpet in to the repair shop and ask them to replace the spit valve cork. This is usually a very cheap and quick repair.