Sinjz, I feel your pain. Had the same situation happen in the past and it was the battery. Like others have said, if the batteries are not forced to power your PC which puts the real strain on them, they often continue indicating they are good. They usually last 3-5 years depending on use. If older than 5 years, plan on it being bad.
I had a series of talks with engineers at APC and read a bunch of sites before I began to realize how crappy and overstated many UPS brands and models really are. I typically bought mine at Staples.
If you are like me, you sort of guessed at your actual needs rather than measuring the amps being drawn by your whole setup while plugged in and running. I have owned about 15 UPS's, and I have to laugh when I remember back to my early purchasing decision was based up the "seems about heavy enough when lifted" test.
APC does have a decent website you can lookup your equipment and get a rough idea of what you need here.
I ended up getting one of these Sperry Digisnaps years ago for pretty cheap, and you plug everything into a powerstrip, turn it all on, then measure the current being used at full load while doing a couple of common tasks. This gives you a reliable indicator of what you need...then you add another 20-30% above that....then figure out how long you want to run without power and pick the model you REALLY need.
I ended up getting their APC Smart UPS 1500 which periodically puts the UPS into being used by your setup, shows how much of the load capability is being used of the UPS capability, and conditions the power coming into your house to smooth out spikes and drops. After about 3.5 years, my battery replace alarm came on, and I'm about a year into the new one.