|"This PC worked fine untill the power suddenly shut down in the neighborhood..."|
I suspect that's damaged the computer.
Power outages, especially sudden ones, often have power surges or power spikes associated with them - if your computer was plugged into AC, even if it wasn't running at the time, anything on the computer or anything connected to it could have been damaged by that or those if it has no decent anti-surge/anti-power spike device - between the computer and the AC source, AND between everything that plugs into the computer that plugs into AC and the AC source, AND that protects the cable connection that connects you to the internet.
If the outage was caused by a lightning strike on the power grid, that can damage the computer, or anything that connects to AC that connects to it, even if it is connected to an anti-surge/anti-power spike device, even if the AC power to the computer was switched off, if the AC cord was plugged in at the time.
Power outage events often damage the power supply on desktop computers.
Check your PS.
See response 4 in this:
You could try another power supply that has enough capacity you know powers a system okay.
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
You could try loading bios defaults in the bios Setup, or clearing the cmos, but if you have the same situation with another power supply that probably won't help.
Sometimes a power outage event makes some connections inside the case iffy.
Unplug the case/power supply.
Power off your monitor.
Open up the case by removing the left panel as seen when you're looking at the front of the case.
Some Dell cases have a latch you must push one way rather than screws you must remove at the back of the case.
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 are all the way down in their slots.
It might also be a good idea to unplug connectors and any cards and examine them for evidence of damage - e.g. burnt or damaged pins or contacts, carbon deposits.
See response 2 in this - make sure the modules are properly seated - remove them to see if any contacts are damaged on the modules or in the slots: