When an APC UPS is not working, it can be a few different things. One of the most common reasons for an APC UPS not to be working is that the battery is not charged. If the battery is not charged, the APC will not be able to produce power.
Another common reason for an APC UPS not to be working is that the power cord is not plugged in properly. If the power cord is not plugged in properly, the APC will not be able to receive power.