Her tail down and hunched up appearance could be several things. She may be having some reproductive disorder, such as salpingitis or coelomitis (formerly egg yolk peritonitis.) It hard to know exactly. It is easier to diagnosis after death with a necropsy, but when they appear sick, we have to observe the crop emptying, the poops, if they will eat and drink, and try treatments. I think she may have already had something wrong, and the lack of water might have pushed her over the edge. If she has not been wormed, I would do that. Most older hens have built up a tolerance to coccidia in the soil, but they can get it if they are sickly. Is her crop emptying overnight? Can you offer some egg and mushy wet chicken feed? Bits of tuna or ground meat might tempt her a bit. Let us know how she is doing. I would also check her vent occasionally, in case she is egg bound. Does she normally lay eggs. Human calcium tablet or Tums given orally can help pass an egg.