Are you sure it's not the battery? The primary failure point in all UPS supplies is the battery. If you don't know what you're doing from the get go, get the unit repaired or replace it with a new one. Batteries are usually cheaper than entire unit's. You could test to see if it was the batteries by connecting a car battery (or two if it's 24V) in place of the existing battery and see what happens when you power it up. Many UPS's are made to be repaired, so even if it is the unit it'd probably be cheaper to have the unit serviced. Check the website of your UPS's maker or contact them via mail, they usually can recommend where you can get their units repaired.