What would cause a car to get stuck in reverse?

Last week I took my car to the mechanic because of engine misfire issues. They diagnosed it as a bad head gasket. I paid to have them fix this and also put a new timing belt in because mine had a crack.

Less than one hour after getting my car back I was going to put my car in Park, but it would only go in Reverse regardless of what gear I tried to put it in. I ended up having to have my car towed back to the mechanic which unfortunately does not open again until Monday. Does anyone know what would cause this issue? Is it likely the mechanic did something to my car when they took the engine apart?

Thanks!