Hi rbfrank,
Thanks for coming to Apple Support Communities for help with Safari today. It sounds like you are unable to open some webpages.
Have you verified the pages work in another browser like Chrome or Firefox?
Check out the steps here: If Safari doesn't open a webpage or isn’t working as expected
You can switch off extensions as suggested in the article or boot into safe mode to see if a Safari extension is preventing loading of the page: Start up your Mac in safe mode.
Which macOS is running? You can find out in the Apple drop down menu > About This Mac. If you have an update available, this would also update Safari and may resolve this issue if it is security related: Update or reinstall Safari for your computer.
Finally, you can also see if the pages load inside a new user account. This would let you know if the issue is isolated to your user or system-wide. Here's how to make one: Set up users, guests, and groups.
Have a fantastic day!