put the transmission into 1 if it'll go, in not 2 or D. Get into the back seat and grab the plastic handle and pull up and back real hard. There's a groove in the round bar and a rib on the platic handle that slots into it. I didn't have the strenght to pull off my T-handle so my husband had to do it for me.
I know on most of these the spring loses tension and the button sticks in but when that happens you can normally put it in all positions. Perhaps the cable is broken that connects the lever the button pushes to the detent rack down on the trans tunnel under the shifter bezel. Or maybe the lever the button pushes on is broken. Anyway, after you get the t-handle off you can see the lever and if its okay, if it is remove the shifter bezel and you should be able to see how it works and figure out what's wrong from there. You should be able to remove the detent mechanism altogether for a workable, if a little crude, fix.