You probably haven’t. Over winding is an expression often used when a clock does not run just after it has been wound up, and may have many causes. It is possible to over wind a weight driven clock by lifting the weight so quickly that it gets wedged into the underside of the clock.
If there is no tension when you try winding again then the spring is probably broken, or it may have come off its hooks. If the spring still gets tight then you may be using too large a key, or some part of the gearing inside the clock may be disengaging.
This can be quite unpleasant. As you wind the clock you usually hear a clicking sound – this is a ratchet which locks the spring as it is wound up. If part of that mechanism fails all the power in the spring comes back through the key.
You might just have put it out of beat. Listen to it – when running, the ‘tick tock’ sound should be even. This can be a common problem with wall clocks which can move when the door is opened to wind them.
Look where the hands are when the clock stops. If they are in the same place (for example 5 past 1 or 10 past 2) then they may be running into each other, and one of them may be loose. If it’s between 12 and 1 then the clock may not have struck 12, and stalled the clock.
A pendulum clock needs to be on a steady and stable surface. It should not rock or wobble or be in a vulnerable location where it could be bumped in to.
Check the battery. A drained battery will corrode and damage the clock so it’s a good idea to change the battery as soon as possible.If it’s good then the movement is probably worn out. Fitting a replacement movement is usually straightforward.