Inspiron 24, Model 3455 suddenly powered off, won't power up

Try this, for starters:

Unplug power cord from *rear of PC*. Disconnect all USB devices, eg printer, etc. Press/hold power button on PC for ~30 sec. Reconnect only power cord to PC and see if it boots now.

Thanks, but I already tried this and it didn't work.  It has a power brick style power cord like a laptop.  I've pretty much concluded this is a failed motherboard, as there are no "power supply" components inside the case.  It's all in the power cord, which delivers 19.5 vdc to the motherboard.

Even with an external power brick, sometimes discharging the capacitors on the motherboard can help with a problem like this.

It doesn't have to be the motherboard. There's also a Control Buttons board in there.  BTW: Are any of the buttons on the edge of the display stuck?

And, physically examine all USB ports with a flashlight after unplugging and pressing/holding the power button. A shorted USB port can cause the symptoms you're seeing. If the contacts are bent and shorted, use something NON-metallic to separate them.

I assume you didn't buy the 3-year extended warranty from Dell, but check with your credit card company. Some of them extend the warranty on items purchased with their card for the same length of time as the original warranty.  So you'd get another year and hopefully they'll pay to repair or replace the system...

I'll pursue the extended warranty through the credit card.  Tried all your suggestions but still no joy.  Thanks!