When a UPS goes dark, not much can happen as that is typically between the outlet and EVERYTHING else.
Are you able to tell if it failed due to worn out batteries?
When servers are ‘in the other room’ where we can’t hear them, their beeps and siren warnings go unnoticed and are of little value. The UPSs that attach the server with a USB connection so that the server can monitor the UPS and send email alerts when the UPS needs to talk to someone can potentially prevent issues like this.
As for brands, there are several major players and my experience is that they are all pretty good with my preferred being APC.
Here is a load calculator and teaching tool. You can take the result and apply it towards any other brand.