To remove a stuck grinder bit, you'll need to get a battery terminal puller.  This is a common automotive tool and will certainly be available at your local auto parts store.  The price might be less than $20.  If your store doesn't stock the battery terminal puller, ask about a similar tool called a gear puller.  You want the adjustable version because the grinder bit is only 1" or 3/4" diameter and the rigid gear pullers have jaws set wider than that.  

Remove the work surface and set the puller over the top of your stuck grinder bit with the legs clamping under the bottom edge of the grinder bit.  With the center set screw set on the top of the shaft of your grinder, turn the set screw to pull the grinder bit straight up the shaft.  It is very important not to attempt to pry the grinder bit loose as that may bend the shaft and ruin your machine.