My carbon monoxide alarm went off once but now its stopped..what to do?

Our alarm is about 6 months old and tonight it suddenly started ringing continusly for about 30 seconds..my partner took it down and it stopped. Put it back up..its quiet now. We live in a bedsit with no gas appliances in our room except a boiler below us in bathroom. Like i said it was ringing for about 30 seconds but now its completely quiet. Called the gas emergency line and got told they're sending someone out. Waiting now. What should we do?? Anyone have any advice on why the alarm went off? Thanks