Youtube has stopped working suddenly

What has stopped working? More detail please. It's not a YouTube issue. see screenshot Let me try my Windows 7 laptop.

https://www.youtube.com/results?search_query=jun+shibata+cover+70%27s

You should update the browser to the current version. It works for me. see screenshot

I am up to date, and it's still nothing but blanks where things should be. I can however run videos. But full screen doesn't work. I'm using windows 7 on a dell laptop that is 10 years old. Why would things suddenly stop working I wonder.

My laptop HP ZBook 17 G2 is from the same time frame and works. Have you recently visited Dell to check for updates to the last one provided for the model?. Also, the current version of Firefox for Win 7 is 115.10. see screenshot

Current ESR --> https://www.mozilla.org/en-US/firefox/all/#product-desktop-esr

A full browser screenshot could be helpful.

You can also reset Firefox. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Can you try under a different user account or you can also create a new profile. see screenshot