I’ve been faced with the same situation, and this is what I did, although I would NOT recommend it:
35-year-old stallion was down in his pasture and couldn’t get up because he decided to lay down on a hill and his legs were pointing up the hill. I was all alone, with no idea when the BO would be back. Watched the stallion for about 15 minutes, until it became clear he was not able to even get up onto his sternum. I took two lead ropes, attached them to his bottom legs, and rolled him over so his legs would be pointing down the hill (I tried it from the uphill side, and realized I was directly in the line of fire if he flailed, so I had to roll him TOWARDS me, which scared the cr*p out of me since I envisioned him rolling down the hill at me). Then once he was pointing down the hill I realized after watching him a while longer that although he was on his sternum, he wasn’t getting up on his own. This was the really stupid part. I got up next to him and pushed against his shoulder with my shoulder as hard as I could, encouraging him to stand up. He finally started standing up but had to lean on me the entire time he got his legs under himself and pushed. Fortunately he was a smallish Arab so weighed maybe 800 lbs. but at that moment I realized how dumb I was and how easily I could have been crushed, all by myself, alone at the barn. I was happy he got up, though. A few months later he went down for the last time. He was the sweetest old boy I’ve ever known and I still miss him.