So sorry to hear about your losses.
Can you offer feed and water in the coop? This is often the safest way to offer feed anyhow, as it can greatly reduce waste from rodents and wild birds.
Your hen is probably, wisely, not venturing out where she saw her flock mates predated. She's scared to go out where it is, in her mind, unsafe. It might be, since you lost two hens! This is a normal reaction on her part and in time she should come out again in time, perhaps several days from now, though you are right in that she will need food and water before then. If possible give her some while she is in the coop.
Your next step would probably be to determine how best to make your birds safer because whatever took your other two hens probably now considers your yard as an open buffet; there is a good chance it will be back. Predators are smart and know a food source when they find one. If you don't take some steps you could soon have no birds at all. A lone hen is going to be much more susceptible to predation than birds in a flock, because she won't have extra eyes (her flockmates) to help watch for danger.
You can consider a penned run, or lots of extra hiding areas. Some people use livestock guardian dogs.
Chickens are social animals, and while sometimes you will get a 'lone wolf' in an individual, in general they are happiest with some flock members. You might consider checking the classifieds and the buy/sell section here on BYC to see if anyone has birds near you that you can adopt. Always use good quarantine methods when bringing in new birds, though, as many diseases can pass quickly from bird to bird and some are fatal. That's a whole other discussion, so if you decide to get more birds, it'd be wise to look up "quarantine" here on BYC and read about how to do it properly and what to look for when picking new birds.
Good luck to you and your hen, and again I am sorry to hear of your loss!
