I assume you have a metal steer tube, either steel or alloy?

If your stem is properly adjusted then it doesnt matter if your compression cap is even on the bike. The only function of this cap is to set the tension on the headset, the stem has 2 bolts that hold it securely to the steer tube. you are in no danger of your stem coming off unless you have a carbon steertube(ie most carbon forks).

If you have a carbon steer tube I'd be concerned as you should not have a star nut in a carbon fork, you use a compression plug. A star nut will destroy a carbon steer tube.

Can you get the compression cap off of the fork at all? If not turn the bike upside down and take the front wheel off.
Next take a long metal rod(a long screwdriver or the like) and put it through the bottom of the steer tube until it contacts the star nut.
Use a hammer to tap the star nut(you may need some force) so that it goes higher up the steer tube, maybe only needs 1mm
hopefully if you can move it high enough it will bite and you can then unscrew it.


Always lube threaded parts before installing them, especially when they contact like metals(ie steel to steel), to prevent chemical bonding or rust from locking them up.