This happened to me with a lone chick I had to raise by itself. I ended up putting the brooder in my room so it could still see and hear me, and then when I had to leave I'd try playing some talk radio next the brooder so she could still hear voices. If you're using a heat lamp, maybe consider switching to a Mama Heating Pad, if possible-- that way it would be dark at night so the chick would sleep better, and it could have something warm to snuggle up to (a cursory 'mama heating pad in brooder' search will turn up a great thread and a handful of great how-to articles-- Blooie's is my favorite)
Those are really only temporary solutions, though. Getting a friend (or two) for it is absolutely the best solution, even if you have to wait a week or two (we got a day-old when my aforementioned chick was almost four weeks old, because that was the earliest they were available, and despite the age difference they became fast friends. If possible, though, stay within a two week margin). One or two of the chicks that hatches from the current broodies would work - just bring them inside with the current one.
Best of luck!