Thats the end link... it connects the sway bar to the strut, you hold the nut with the allen key and wrench it with a 14mm or so open end. Its a ball and socket joint with a grease filled bushing.
If you actually severed the bolt your car is not safe to drive.
Be very careful turning! you could roll the car very easily.
If the bolt is still holding everything together and its just clunking around, all you did was rip the bushing.
Either way you'll have to replace the whole endlink.
CTM is
www.ctmotorsport.com. The owner, Sean aka soundlzrd, is a TN member.