L.P.
Hello, Kristin! I'm sorry you're struggling with this--sleep issues can be so frustrating for everyone!
First I would look at the possible physical causes--an undiagnosed sinus or ear infection would cause pain lying down and could make a child reluctant to go to bed as you describe. Neither of these conditions always has other manifestations to clue you in that something is wrong--my neighbor's daughter had double ear infections and the only clue they had was mild irritability!
If the problem is more of a separation anxiety issue then it too can be managed. My oldest would always sleep by herself if nothing was wrong, but my middle child had horrible bouts of sepparation anxiety. We learned the hard way that if he was left crying in his crib for *any* amount of time the anxiety about bed would sky-rocket. With him (and he's almost 3) it's still the case that we will occasionally go through periods of time when one of us needs to sit in the room with him while he falls asleep. The first few nights we might need to stay there until he's asleep but gradually we can decrease the amount of time we spend--tell him that we'll be back to check on him soon etc until he's once again comfortable just going to bed by himself.
This is a lot more work, obviously, but for a kid with separation anxiety it's crucial that they feel secure and cared-for in bed if you want them to go there willingly. Once our son is comfortable again after a period of anxiety he'll even ask for naptime! But it takes work to get there which can be even trickier if you have other kids.
For now be patient with her. If there's nothing physically wrong try keeping her company for a few days. I wouldn't talk or take her out of bed, just sit next to her quietly. Rub her back for a few minutes or rock her first--whatever helps to soothe her--and then just be there where she can see you. I used to bring in a book and booklight to read while he fell asleep.
Good luck! I know it seems like a step back but it really won't last forever!