Several methods:
1. Look around for the name of the contractor that installed the system. Many times there will be a business card taped to one of the main system components. Unless this contractor is out of business, unreliable or over priced they will be your best source for repairs, upgrades or replacement.
2. Google the manufacturer and see what resources are available. Everything from distributors, contractors, installers and documentation might be a few clicks away on the web.
3. Check connections and power supply. These ate the two main reasons electronic systems don't work.
4. Post the make and model here along with a good description of the problem. There are quite a few knowledgeable people here that might be able to point you in the right direction.
Good luck!