|"Gateway desktop / E-2100"|
".....last week we had a storm that dropped power for three full days...."
Power failure events can cause power surges and / or voltage spikes that can damage anything on your computer or anything connected to it at the time.
If the event was caused by lightning strikes, that can damage anything even if you have the computer and everything plugged into it plugged into something that protects it against power surges and spikes.
If you are fortunate, only the power supply was damaged.
If you have another working desktop computer you can borrow the power supply from, or if you can borrow one from a friend's computer, try connecting that to your mboard. Preferably it should have the same (max output) wattage capacity rating as on the label on your power supply, or higher.
If you're lucky, you'll then find your computer will work fine.
If that doesn't help, something else has probably been damaged, most likely the mboard.
This is much less likely...
Sometimes when you have a power failure event it causes wiring connections or components inside the case to develop a poor connection.
Unplug the case/power supply, or switch off the AC power to it otherwise.
Power off your monitor.
Open up the case by removing the left side panel as seen when you're looking at the front of the case.
In addition to the following, unplug all wiring connections, cards in slots, ram modules, one by one, examine the connectors or sockets or slots to see if they show signs of damage, plug them back in.
Check all the connections of the wiring to make sure they are all the way onto their pins and into their sockets, especially the main connector from the power supply. The wires close to the mboard going into the main power connector/socket should be more or less perpendicular to the mboard surface rather than at an angle. Make sure all cards in slots and ram modules are all the way down in their slots.
"power cord for computer was tested"
Unless you live in a country or region (e.g. the UK) that requires a fuse to be in the wall plug end of the cord to the power supply and the fuse in that has blown, it's extremely unlikely there's anything wrong with that cord.
If your computer is no longer usable, in most cases, the hard drive and the data on the hard drive have not been damaged.
You can connect the hard drive to another computer one way or another - just don't boot Windows from your drive on the other computer - and copy the personal data you don't want to lose to elsewhere.
If you have personal data you don't want to lose, you should have a copy of that on at least one other data storage device, or on a hard drive partition that is not on the same physical hard drive ! !