|It sounds like your UPS and computer experienced a power failure event or another event that produced power spikes or surges, or a short. If the event was caused by a lightning strike on the AC power grid, or a short, or by the original power supply failing destructively, that can damage anything connected to the computer even if the UPS has anti-power surge and anti-spike protection. |
Try the computer without connecting it to AC power via the UPS.
If that doesn't help and you're SURE the power supply is connected correctly, the mboard, possibly other things too, is probably damaged.
If the power supply is an el-cheapo, some brands are well known to be a lot more likely to damage the mboard or other components while failing - e.g. BESTEC power supplies, used in most emachines desktop computers and in some cheaper models of other brand name system desktop systems, have that reputation.
You have to plug in the computer and everything connected to the computer into something that protects it against power spikes and surges - everything that plugs into the computer that plugs into AC directly or via an AC to DC adapter, AND the cable that connects you to the internet, if you computer is connected to a router or to a combo modem / router via a network cable, AND the telephone line if you're using a dial-up modem on the computer or are using an external dial-up modem. UPSs usually do not have enough plug ins for that, on their own.