Did you put the decking down yourself? If you did, you should have made some part of it removable (eg, a portion that can lift up, like an attic trap-door) for just such a reason. If you didn't leave this access, I'd recommend that you make it now - then try and fix the drain yourself if you can! If a carpenter did the deck for you, you could ask him to come back and remedy it so that you now have this access, cos you need it now, and you're definitely going to need it again sometime! Then fix the drain as above. If you get a drain cleaning contractor in to fix the drain, he may need to get access too, and one way or another, that access will eventually be required, so you may as well start with that! Good luck