I like Charlie's idea. The vibration of driving around might slacken the mechanism. If not - and if access is possible ( I don't know your particular setup) - try giving it (the inertia reel) some repeated light taps whilst giving the belt a jerk at the same time.
Is the belt tight? If so it may not be possible to free it without introducing some slack and that will probably require undoing either the end of the belt where it's secured or the inertia reel itself.