Did you remove the goal off the experiment, or delete the goal completely? If you removed it, then you should be able to just re-attach it to the experiment. If you actually deleted it then I think you're going to have to put in a support ticket to get it back. I believe the actual goal data is still there in the Optimizely database, but you just can't access it any more. You can try recreating the goal identically, but I'm not sure that will allow you to actually get back at the data.