My take is that the bolt head has broken off, leaving only the threaded portion of the bolt.
If it's flush, you'll need to drill it out, if it is sticking out, you may be able to get a grip on it or you may be able to weld a nut to it.
If you still have others to remove and you re afraid that they are also going to break, here's a trick to get some penetrating fluid to the threads.
On the head of the bolt, drill a really small hole at the 12:00 position, through just the head of the bolt.
Now you can insert the tip of your favorite penetrating fluid into that hole and give it a squirt. The fluid can now get to the threads.
Worked great on the windshield frame bolts of our '03.