well how soon till the lights go out... it could be that he is just disorientated (shutting the lights off would help that) or maybe he got hurt by something or got spooked.(if your sand has lots of broken shells in it, it is possible that the goby got scraped by one. I got to go to sleep but I think that if he isn't being bullied and the water is good I would just wait till tomorrow.(if you have a top for your tank you might want to put it on so your fish doesn't jump out the first day, then take it off tomorrow) I'm not too sure what else you can do if the water is good... maybe try a bit of food... let me know how you make out.