What is probably happening

What to do

Dead batteries

The VRLA batteries in small UPSs aren't very hardy. Keep the UPS on the floor and in a cool area with plenty of circulation. When the power goes out, shut down your PC as soon as possible. Use the UPS to allow power glitch ride-throughs and orderly shutdown, not to keep working when the power goes out.

OR

Power infrastructure problems may have zapped the batteries--Have a power quality engineer do a site power quality analysis, to determine if you need to repair your power distribution system and/or grounding.

UPS and power backup items.