I've spent some time already looking through some help files, but I can't seem to fix my problem. Right now, whenever I try to launch Dark Messiah Multi-player from Steam, it says, "The game is preparing to launch", then this is immediately followed by "The game is currently unavailable." This game has been working for me up till now, and I hope that someone here can help me fix this problem. Thanks to anyone who might have the answer.

Could be because of temporarily server problem at Steam (rare, but it happens), could also be one of the stations between Steam and your Internet Provider, or your bandwidth limit exceeded (if you have a limited account)...

Try deleting your clientregistry.blob file located in your Steam folder. Restart steam and see if it works then.