I have a Back-UPS XS 900 that began flashing the overload light, with a constant warning tone. This happens with or without anything plugged into it, and started happening with most of the computer equipment powered off. The serial number is JB0717000165. Is the unit still under warranty?
i believe this indicates a fault condition. the UPS indicates it was manufactured in early 2007 so it would have a three year warranty. i estimate your warranty ran out mid-2010. if you do have a proof of purchase that indicates you bought it later than 2007, then APC will honor the warranty for you.
it is also still an option to contact APC support if you'd like to see if they can do anything for you but I don't believe this condition can be cleared, without coming back in a very short amount of time.
you can braindead the unit if you want and try that - turn off and unplug all load on the UPS (which you may have done) and then turn off and unplug the UPS. then remove the UPS battery and hold down the power button for 5-10 seconds. reconnect the UPS battery and reconnect the UPS to AC power. turn it on and see what you get. even if this works, as i mentioned, i think the condition will not clear or may come back so you may want to keep an eye on it.