The first step to solving any problem is to understand it. Think about and be prepared to answer questions like:
What exactly is the symptom?
Under what conditions does it occur?
Is is constant? Intermittent?
If it is not constant, what are the patterns to when it occurs?
Does it happen on multiple sources or different kinds of signals?
Look for the obvious. Consider the possibility that it could be user error. Even the most experienced of us have chased our tail because a tape monitor switch was on or a speaker selector got pressed accidentally.
Gather your information. Look for your manual, receipt, and remote and have them ready. Get the brand, model and serial number, along with any chassis numbers and date of manufacture off of the back of the unit.
Look for resources. Some manufacturers have very good online support. Check the Manufacturer and Vendor forum for contacts and other information. Look to your dealer if possible. People who are familiar with your product are the ones you want to be talking to. Look for relevant threads that might already exist in the forums. When you post a question or a problem, be sure to give as much information as possible from the items suggested above, or any other context that might be relevant, such as the other components in the system.