If you have a section or sections of your sprinkler that is not functioning, we need to look at a couple of things. 

First of all - what time of year is it and are you starting your sprinkler for the first time? 

      The most common problem we hear about early in the spring (think March and April) is someone excitedly firing up their sprinkler on the first nice day and either nothing happens or only the zones in the full sun areas of their yards are functioning. More than likely at this time of the year, the sprinkler has been fired up a little too early and there is some thaw needing to happen yet. Wait a couple weeks and try again. 

If your sprinkler has been running and you have a zone that has quit working entirely with no water coming out, you most likely have a valve issue, but it could be a wire issue.

If you have some water, but low pressure you either have a tree root restriction or a leak in that zone. 

If you have sequential zones not working, first check to make sure your controller modules are clipped in tightly inside your controller.  If the modules are all snug, have you done any digging in your yard where you may have cut a wire? Did you clean debris out of your valve box and maybe disconnected the common wire? Is your controller counting down the minutes or is there an error message?

Most of the fixes applied here are beyond the scope of the average do-it-yourselfer but knowing the answers to these questions will help your service technician.