Sometimes it happens. You go to a write-in and maybe you got the time wrong, or the day, or the place, or maybe everyone else did. Don't fret.
Remember, that's the main reason for going to a write-in and you can do that alone.
Also, you may not be alone. You just may not have found the other writers who are there. (This has happened to me several times, and I do my best to look out for other writers, but one year there was a table of what I thought were old friends meeting at Caribou. Nope. Turns out they were there for the write-in, but the first person started greeting people and they joined her, not realizing we were 20 feet further back. We had a good laugh when we realized we were all there for the write-in.)
Be sure to put something near youthat identifies you as a member of NaNoWriMo. Believe me, it's not easy to tell writers from other people during November. I usually have a simple blue sign that says, 'Write-in' or 'NaNoWriMo Write-in.' You don't have to meet the eyes of every stranger that walks in, just have something next to you that identifies you as being at a write-in.
Oh, if someone who isn't a writer starts asking questions,say, "I'm afraid I can't answer questions until after November. I'm busy writing my novel."
Also, note that the library-based write-ins all feature reserved rooms. I can guarantee you that you will not be all alone at any of the Naperville Public Library write-ins I am hosting :-) Ditto for Catherine's (Cee-Bee) Woodridge Public Library ones. But Katherine makes a great point that write-ins are intended to be productive writing times.