must be one of / many of multiple issues
1 - your ups is not charging the battery..
2 - one or more of your battery cells have gone dead..
3 - low voltage
4 - Miscellaneous technical issues
easiest way to check battery
1 - measure specific gravity with a hydro meter wich is easily available for 150rs.. it should be equal and 1.25
2 - measure battery terminal voltage using a proper DC voltmeter.. both when battery is dead and when ups says fully charged
easiest way to check ups
1 - use clamp voltmeter to check if there is charging of over 5 DC amps from the red or black wire connecting to ups
2 - check the fuse inside the ups.. they could have blown
then get the item at fault repaired accordingly