First, get the sheep sitting upright
A cast sheep has rolled or slipped onto its side or back — frequently into a hollow it has worn in the ground — and cannot rock itself back onto its feet. The single most useful thing you can do is right it. Approach calmly from behind, take hold of the fleece or a front and back leg on the upper side, and roll the sheep towards you and up so that it ends sitting on its breastbone with all four legs folded beneath it in a natural resting position. Then stay with it. A sheep that has been down a while is wobbly, so prop a straw bale or your own knees against its side to stop it toppling back, and give it a few minutes of support before you test whether it can hold itself. If it wants to stand and can, let it rise in its own time and watch it for a while rather than walking away.
Why a sheep on its side deteriorates
Lying flat is genuinely dangerous for a sheep, which is why this cannot wait until morning. A sheep's rumen keeps producing gas, and in a normal position the animal burps that gas away; trapped on its side, the gas builds up, the rumen swells and begins to press on the lungs and the great blood vessels, so breathing and circulation both suffer. Add exposure if the weather is wet or cold, the risk of birds or dogs worrying an animal that cannot flee, and simple exhaustion from struggling, and a sheep that was merely stuck an hour ago can be in real trouble. That is also why a collapsed sheep — one that is down for a reason other than being cast, perhaps ill, injured or close to lambing — needs the vet regardless of how it responds to being sat up.
What to observe while you wait
While the sheep settles, gather the details the vet will ask for. Watch the breathing: is it fast, laboured or noisy, and is the left flank blowing up tight like a drum? Look at responsiveness — a bright sheep tracks you with its eyes and ears, while a dull, unresponsive one is a worse sign. Check for obvious injury, for a swollen or prolapsed rear end in lambing season, for flystrike under the fleece, and for anything the flock has had access to, such as a broken fence into rich grazing or a feed store. Note how the rest of the flock looks, because if one sheep is down from a shared cause, others may follow. Write times down, even roughly: when you found her, when you sat her up, and how she has changed since.
What not to do
Resist the urge to dose her. Do not drench a down sheep with anything, offer feed or water to an animal that may not swallow properly, or give a stored medicine because it helped a different sheep last season. Do not drag her by the legs or fleece across the ground, and do not leave her propped and unattended for long stretches — a sheep that slips back flat while you are gone is back where you started. Keep dogs, children and the rest of the flock at a distance so she stays calm, and if she cannot hold the sitting position at all, keep her propped and tell the vet that on the phone, because it changes the urgency. Write those observations down in order for the vet, and if a detail escapes you later, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.