Disk brake brakes work poorly after annual maintainance The disk brakes practically don't brake anymore. If I pull the brake lever as hard as I can, the wheel still turns if I push the bike forward.

Is this normal? Do they need a "brake in" period?

UPDATE: It's a mechanical brake, and the levers are properly adjusted. If I pull the brake lever, I can see the pads pressed against the rotor, but the wheel still turns slowly If I push the bike. This was not happening before.

The maintenance was done at a local bike service, I don't know exactly what they did to the brakes, but the rotor seems to be oiled up.
If the pads have oil on them they need replacing. The discs can be cleaned if done properly and carefully. If the LBS did the work, take the bike back and they should put it right. 

From the sound of it, I would also be looking for another LBS. If they cannot service brakes and keep the oil off they are at probably sloppy - but even the best shops have the odd incident. However, as they returned the bike to you in that condition, it is dangerous, they have let poor work out the door. My future custom with them would be in serious jeopardy.....