Regressing to the Null?

Ok, a bad pun -- usually we say to the 'mean'. However, Callen has regressed to not using the potty at all since she was sick last week. I don't know what to do. She won't even try to go to the potty. I don't want to force the issue and make her want to use the potty even less (if that is actually possible). She was doing so well, especially at school. Any suggestions for how to reverse this negative trend?