When I disconnect the Ethernet cable my desktop now can't see any WiFi

Hi, all. I'm a newbie here, though I was using IBM-PCs when MS-DOS was king. (a LONG time ago).

I have a Win10 desktop system, wyhen we first set it up, we commected it to the WiFi signal coming from our ATT cable modem, and it worked fine. We could access the internet and all was well.

After a few years, I ran an Ethernet cable from the cable modem directly to this computer, in hopes the Ethernet to this computer would run a little faster. It did seem to go little faster, and I could still access the Internet just fine.

We finally got a newer Win10 desktop system, installed it where the old one had been, and plugged the Ethernet cable into the new one instead. Moved the old one to an upstairs room (which had no ethernet cables), set it up there, and tried to log on to the cable modem's WiFi signal.

And found that the old computer now couldn't see any WiFi signals at all. It didn't seem to know what WiFi was. Troubleshooting, just provided the "answer" that the ethernet cable was disconnected, and I should plug the cable back in.

But years ago, this computer ran just fine on the modem's WiFi, with no ethernet cable plugged in at all. Now it doesn't seem to know WiFi exists. Everything else works fine (Microsoft Word, Excel, Command Prompts etc.)

It's as it there was a magic switch somewhere, where it can choose between "Log On To An Existing WiFi Network" and "Operate only off the Ethernet cable and don't even look for a WiFi signal". And that magic switch somehow got switched to Position B at some point.

I looked at the keyboard at the function keys across the top, hoping to find one with a litt e "connection" symbol on it next to the F3 label or whatever. None of them had any such symbol. Tried to press a few, none ever seemed to have any effect on recognition of WiFi.

What did I accidentally trip or switch or unplug, during the last few years while the Ethernet cable was plugged in? Now when I unplug the Ethernet cable, suddenly I have no Internet and no apparent way to connect to it, or even find it.

Help?

Thanks all!

Could be several things.
How confident are you opening up the computer (remove/unplug power, hold power button down for 30 seconds to drain the PSU, open case, hold onto metal of the case while you clean out/reseat the wifi module)?

Check Device Manager (Start...Run...devmgmt.msc)
and see if there are any errors/problems/warnings (especially under the network adapters section)

Visit the PC manufacturer's website and download/install the latest W10 compatible WiFi drivers for your system.
You'll either have to use Ethernet - or use another computer to download the installer (then copy to a flash drive and manually bring it to the PC)

Most likely the new computer does not have WIFI. It would have stated in the Spec’s when you purchased. If it does, it may require an external antenna.

Is the WiFi turned on? Is the WiFi card working and just not finding a signal? If you know what WiFi card you have you could try downloading a updated driver onto a flash drive with the new computer and installing it. Could the distance from desktop to the modem be to far for the signal to reach?