Most of the time (but not always) while browsing the web in safari on an imac, the "Previous page" button goes dim and I can't return to the previous page.
Usually when that happens, the 'next page' button darkens. And you can then go backwards and forwards !!!
If you press cmd + left hand bracket (next to p) that takes you to the previous page. Similarly if you press cmd + right hand bracket, you go to the next page.
Maybe the new page opened in a new Tab or Window, so there is nowhere to go back to!
Try the History Menu to go back.
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