Dell Optiplex 390 Will not start

Buy a $2 coin cell battery in any store and replace the old one. Then see if it will boot. Instructions in the owners manual here.

That should be the first step.

I take it no lights whatsoever, no number lights either.

The PSU is dead.  Not unusual for a PC of that age.  I'd still double check electrical connections.

I've often suggested replacing the size 2032 CMOS battery myself.  Sometimes confusing error codes can be thrown and a new battery will clear it up.  But even with a low or dead battery, you'd still have something when attempting to boot, like a "low voltage" alert for one.

If we go further, it'll help to know what size your 390 is - MT, DT, or SFF?

Thank you, I will work more on that tomorrow, it is a SFF by the way.

Ok, so it works. Just now the time and wifi will not work. I cannot connect to the wifi. Any suggestions?

Glad you got it working.  So which trick worked?

Time can be set in BIOS or Windows.  If time is resetting every time it's turned off, time to replace the battery.  I'm sure you know time updates when connected to the internet.

Next, you may need to reload driver for your USB dongle or card, depending how you're connected.  My previous WiFi adapter recently died.

All I had to do was change the power supply with a cheap one of Amazon.

P.s. if into devices, my wifi will not show up. And it does not show a wifi tab in settings.

Also,  did some work looking at my cables, and my HDMI cable has ethernet, should I just use a VGA cable?

Sorry for keeping this going, but this is currently my veiw