This is tricky! You need the plastic tool that came with the VESA mount and something to snag the tang that’s slipped inwards to pull it out. There is a latch which prevents the case to go back to far which is now on the wrong side. Refer to the sheet that came with the VESA mount to guide you where to slide the wedge in. If you don’t have the sheet you’ll need to search for the sheet on Apple’s site (it’s a PDF).