The chains of my bike are clicking and the gear don't work anymore
If the gears are making noise when you're pedaling it usually is a symptom of the derailleur being out of adjustment.

Two Most Common problems:


Basic derailleur adjustment. In many cases the cable has stretched or something has caused a change in the derailleur cable tension. This is easily fixed with the derailleur cable adjustment barrel which is either on the rear derailleur, or for mountain bikes it's on the shifter body.  Here's a video which will walk you through how to make tune the shifting. 


Video On Rear Derailleur Adjustment


Derailleur hanger has been bent due to an accident or the bike falling over. The hanger is where the derailleur is attached to the bike. A special tool is needed to check and/or fix the hanger. 


There's other stuff which could be wrong but I'd need a better description. Pictures would also help in guiding as mountain bikes have slightly different parts so the details on how to fix will vary.