The threads have stripped out, if the plug will not screw back in.You can get a repair or a Heli-coil kit at the auto parts store.If you are not very mechanical,I suggest having someone else repair this for you.It is possible that the threads are not stripped out-but not likely.
If my info has helped you in any way please hit the accept button, and remember bonuses/tips are always nice. If you have any further questions don't hesitate to ask. Thank you.