So unless you've seriously overloaded them, or soaked them in water, or poked out the cones, you need a new sound card.
Are they internal or external?

were frizzing out now and then. they go bad

What operating system?

If they work on the other machine, plug in some earphones or spare speakers and see if they work. If they don’t, then problem is either jack, or sound card, or audio drivers. Delete and re-install audio drivers.

Ditto any jacks. Be patient. Alternatively get new speakers.

Typically on a PC it is colored green. Obviously have something playing like youtube or music or something. Sound comes through headphones obviously computer is not the issue.

Also possible ... if your phone has a headphone jack, plug the speakers into the phone and play something again like YT or music. Speakers make appropriate sound, speakers are not the problem.

What sort of computer/OS etc?

Windows 10 -computer has a headphone jack and I can hear things so off to the store for a new set of speakers............

Also you should plug phones into the exact output jack you were using for the speakers, not a headphone jack on the front panel.

I read you comment; started a music video,, and pulled the phones out of the from of the PC to plug then in like you suggested, and the speakers work again!

and/or the wires that run from that jack to a plug that's on your motherboard (or sound card if you have an add-on sound card).

That jack, and those wires are often chinzy and prone to problems. If you get any kind of shorting out, it can affect the sound system as a whole.

If it gets too problematic you can disconnect that, once you know where to look it's like a 30 second job. I could help you if you took a pic of the inside of the case ... if that time comes

Sometimes it's because a game is in the background and I have the sound muted in the game. Apparently this affects any sound on the computer at all.

Tonight I was having a problem and during my troubleshooting Firefox popped up and wanted me to restart it for an update. Since I was about ready to restart the computer, I said OK. Only Firefox restarted but the sound was back. Maybe something in Firefox was blocking sound from online videos. I don't know, but it's working now.

several things I hadn't thought of. In terms of cheapest/easiest thing to do I'll delete and reinstall drivers to see if that works. When I pull the speaker jack out and touch the entry point I get static from the speakers. I also got static when I plugged ear buds into the PC directly but no sound.
Time for sleep, I'll check some of the other ideas tomorrow.
Thanks again!!!!!