I'm not sure I 100% follow you here, but I'm assuming you mean I'm playing a round as the only staff on? While I try to avoid that for reasons exactly like this, I'd probably cryo myself to better handle things until more of us were about. It's definitely easier to manage things as an observer.
I'm pretty sure there's a nice list that auto updates for every active ban, so my first step would be checking that to see if their ckey is still there. If not, someone probably just forgot to add a note about it being removed. If someone somehow forgot to BAN them and only added a note though, then I can finish that off for them after informing the player what happened.
Ask them for a screenshot of the error it's giving them, check ban list in the meantime to see if I can figure it out myself, let them know the reason they were banned if there is one, or if it was a sticky. I'll send them a screenshot with an arrow pointing to forms too, if they have trouble finding the spot to ban appeal.