Harmonica Out of Tune? 
What to Do?

All harmonicas eventually go out of tune, some faster than others, according to the kind and make,
but most often harmonicas go out of tune as a result of the way that they are played.  Quite often they are played too hard,
but even the most well-kept harmonicas will go out of tune eventually.

Anyway, what to do when your harmonica goes out of tune?

Here's some ideas, from easiest to hardest:

1. Get another harmonica
2. Keep the harmonica as one you can learn to work on harmonicas with, and get another harmonica
3. Sanitize the harmonica and give it to a child to have fun with, and get another harmonica
4. If it is an expensive harmonica, send it to a repair service
5. Learn to tune harmonicas yourself

If you are learning to tune your own harmonicas it is good to have planned ahead and used option #2 where you've got a stock of harmonicas to work on, because tuning harmonicas is both an art and a science and you will be ruining some reeds if not ruining some entire harmonicas in the learning process.
 
Tuning a harmonica involves, put as simply as possible, scraping or scoring the metal reeds to raise or lower the pitch.  You have to remove the cover plates from the harmonica, determine which reed needs tuning, and use an electronic tuner to see where the pitch needs to get to.

One of the things that complicates this is that if a harmonica is tuned to exact pitch for all 20 reeds it most likely won't sound very good when playing chords.  And when tuned for playing chords it is tuned as a compromise between single notes and chord playing, and that is a compromise that takes some tweaking.

So if you really love to do stuff like this,when your harmonica goes out of tune, then you can get into tuning your own harmonicas.
If not and you just want something that sounds good as soon as possible, then go with option #1 above and get another harmonica.