As the xbox controller becomes more and more complicated over the years, it gets more and more likely that your controller will develop a fault. If your control pad is still within warranty and hasn't been repaired then you can send it back to the Microsoft Xbox Controller Repair centre. If however you have had your control pad longer, or you have had a xbox controller mod then the warranty will be invalid, in which case you will have to send it away to an xbox controller repair company, or try and fix it yourself.
Depending on the fault, you may be able to fix it yourself just by searching for "Xbox Controller Repair" on google or youtube. However please make sure you find the right guide as there will be different insides in official, to unoffical control pags, and also differences between wireless and wired cables. If you have had an xbox controller mod fitted than this makes it even more difficult to perform an xbox controller repair.
If a DIY xbox controller repair isn't possible, then it may be worth just buying a new xbox controller. In which case check out this section: Buy Xbox Controller