If the garage door disconnected from the opener accidently, then the problem is most likely with the components which link the two components into one system. These are the arm which connects the trolley to the door, the trolley itself and the rail. The disconnection may be due to the fact that one of these components is damaged or broken. It is also possible for the issue to come from the drive which moves the trolley back and forth, but this is less likely.
In general, there is an emergency release mechanism which is used for disconnecting the door from the opener if there is a need for this. In order for this mechanism to be engaged, the specially designed handle has to be pulled down. If someone has pulled down the handle accidently, the two system components can be reconnected with its use as well. It has to be pulled at an angle.