Like all instruments the harmonica has occasional problems which need maintenance. Fixing harmonica problems (at least most of them) is within the reach of all players, with a little guidance.
Harmonica note blocked: If a note which previously worked now refuses to sound, then something is caught in the reed. This is often just excess moisture from your mouth. Tap the harmonica mouthpiece firmly on the palm of your hand, then breath in and out rapidly over the blocked hole. This will usually remove the excess moisture and solve the problem.
If the note remains blocked then something solid is usually caught in the reed, often a small hair. Take a wooden toothpick, place it into the hole with the blocked note and push gently on the reed which is not sounding. Remember the blow reeds are the ones you see when looking into the harmonica hole.
If the note still remains blocked, then remove the outer plates of the harmonica by unscrewing the bolts at each end. Once the outer plates are off, place the bolts inside one of them. Then inspect the reed which is not sounding, you should be able to see what has been caught in it. Push the reed out gently with the wooden toothpick, much easier to do with the cover plates off. The object caught in the reed should fall out, solving the problem.
Cannot get the two hole draw note to sound: If you have never gotten a clear note from the two draw hole, do not be discouraged. Most beginning players have trouble with this note. Detailed instructions for getting a note from the 2 hole draw are here. The problem is almost certainly with your technique, and not the harmonica. Be patient, keep practicing, you will succeed.
Harmonica note only works with a light breath pressure: This problem usually occurs with blow notes, and happens when the "gap" between the tip of the reed and the reed plate is too small. To increase the gap (and solve the problem) remove the cover plates, take a wooden toothpick and press gently on the middle of the reed, pushing it away from the reed plate. Remember that the blow reeds are on the inside of the hole, the draw reeds are on the outside. Push the reed out five times. Now hold the cover plates over the harmonica (don't bother with the screws) and blow (or draw) hard on the reed. If the note still blocks at high breath pressure, then repeat the process. The gap only needs to increase slightly for the problem to be fixed. Be sure not to push too hard on the reed, you may damage it.
Harmonica note sounds bad: If one of the notes on your instrument suddenly sounds bad, then it is probably is bad. Harmonicas do not last forever. Eventually one of the reeds will develop a slight crack, the note will become lower, the instrument will sound bad as a result. If played regularly a harmonica should last between 6 months and a year before this happens. Unfortunately, once this problem occurs your harmonica is broken, and must be replaced. It is possible to replace the broken reed, however this is a task for a skilled technician. The cost for installing a new reed usually exceeds the cost of a replacement instrument.