This was originally posted on APC forums on 7/30/2016
The multiple flashing lights are indicators of an internal fault. (The number of illuminated lights varies depending upon the fault.)
In most cases the problem is a sticking or burnt-out (welded) relay. Technically speaking, a repair is possible but you will be working entirely on your own and there is very real danger in working on these units. The units are not considered repairable.
Sometimes smartly rapping the relay casings will cause them to become unstuck, allowing a successful startup and confirming the problem. Do this only with the unit completely disconnected from all electrical supplies, including batteries...if you do it at all.