Go step by step and test.
1. Click in the menu bar and choose “Force Quit”.
Select Safari and click “Force Quit at the bottom of the Force Quit window.
Hold the Shift key down and launch Safari.
2. Empty Caches
Safari > Preference > Advanced
Checkmark the box beside "Show Develop menu in menu bar".
Develop menu will appear in the Safari menu bar.
Click Develop and select "Empty Caches" from the dropdown.
3. Delete Cookies
Safari > Preferences > Privacy > Cookies and other website data:
Click the “Details” button.
Remove all cookies except ones from Apple, your internet service provider and banks.
4. Power off the router. Unplug it from the wall. Wait a while.
Plug it back to the wall. Power the router on. Wait until all lights are lit properly. It will take a while.
Restart the computer.
Start up in Safe Mode.
https://support.apple.com/kb/PH18760?locale=en_US