UPS constantly sounding an alarm tone

I bought this UPS for our NGO office. It started to make a high pitched tone (as if it were overloaded) almost from the start. The tone goes starts from the moment you switch it on, even with no load on it, and will continue even when it the UPS is switched off. The only way to stop it is to hold the off button in for 5 seconds or remove the battery connector from the rear. Unfortunately I installed it the evening before I went on leave, and came back to find that the alarm had been going for almost 3 weeks, and that the cleaner had thrown away the box and all documentation, including the software. Obviously it cannot be used while making the sound unless you are looking to give yourself a serious headache. Suggestions please.