Did I kill something in my electrical system by leaving the key in the on position overnight

Howde ! My 91 Toyota pickup is having a small problem, or rather I am. I left the key on he other night in the on position, not start. The next morning it started fine but the brake light and battery light are stuck on in the dash. My truck also overheated, and is low on liquids again after topping it off. I haven't checked for hot in the pipe yet, so I don't know if it is my thermostat yet. I am bringing my battery to the store to check for a bad cell. Any ideas as to how much damage I did?