It won`t charge but It can only use Battery power.
I have the same problem.
I didn't use mine for like one year and switched it on today.
Suddenly the circuit breaker turned off and once I turned on, the UPS stopped charging on A/C power but runs only with remaining battery power.
I think the PSU of my desktop got malfunctioned after the circuit breaker disconnection incident.
UPS : APC 650VA
Power Supply of the Desktop PC : Raidmax 500AF
Still looking for the UPS circuits but in the meantime do the paperclip test on the PSU to check if it is turning on OK and supplying all the voltages as shown for the test or whether it has some problems