Yes, send the certified letter, etc. But if you get no response, contact the County. Montgomery County has an Office of Consumer Protection and assists with situations like yours. Often they are already aware of the contractor, and can make arrangements to get the contractor to complete the work in a timely manner. If they are completely MIA, but are licensed, there is a fund that can reimburse you for your losses.
From the website....
IF YOU DO BUSINESS WITH A LICENSED CONTRACTOR, YOU ARE PROTECTED BY THE HOME IMPROVEMENT GUARANTY FUND FOR LOSSES UP TO $15,000 THAT RESULT FROM POOR WORKMANSHIP AND OTHER VIOLATIONS OF THE HOME IMPROVEMENT LAW. YOU ARE NOT PROTECTED BY THE GUARANTY FUND IF YOU DO BUSINESS WITH AN UNLICENSED CONTRACTOR. This is the most important reason why you should only do business with a licensed contractor. If losses caused by a single contractor exceed $50,000, each consumer receives a pro rata refund.
Here's the link...
http://www.montgomerycountymd.gov/ocptmpl.asp?url=/content/ocp/consumer/guide_home_imp.asp