If your slide stop is not all the way removed, you need it out in order to move the slide back. Once it is removed, open the action back to the disassembly marks and insert the slide stop allthe way in. Then you can once again open the action and push the ejector forward with the slide stop holding the action open. I did this once and it worked for me.
If the slide stop is already totally removed and the slide will not move back, I don't know how to advise you. I hope this helps.