Without knowing which light is flashing I can only guess. From my experience and assuming it is the battery light that is lit and NOT the overload light, it can be one of two things, one is your battery is near the end of its life and needs to be replaced, and the other is that your battery is improperly connected.
To confirm if your battery needs to be replaced or not, you can run a self test. Ensure that the battery has had time to fully charge (and that it is definitely correctly connected), and then turn off the UPS, and turn it back on. It will automatically run a self test.
if the light stays lit, you need to order new batteries. One source is here: http://www.refurbups.com/APC-Back-UPS-RS-900-BR900-Replacement-Batteries
If it IS the overload light that is lit, you simply have too much equipment attached to the UPS, more than the inverter can handle. Try removing some equipment until it turns off.