The hysterical customer is usually someone who feels that there is no one on "his side", he has reached the end of his rope, and you have been unfortunate enough to be at the tail end of what got him there. You may not have been the cause of all his problems, but he is nevertheless looking to you for the solutions.
Dealing with him is simple. Be patient. Listen intently. Acknowledge your understanding and express empathy.
Try it. Even if you can't solve his problem, you'll be surprised at how positively he will respond.