I've had a MacAir for two years; no problems. Suddenly today, it will not allow me to log on to a site I use regularly. Site is okay because I can get to it with my iPad.

Go step by step by step and test.

1. 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 the lights are lit properly. It will take a while.

Restart the computer.

Start up in Safe Mode.

http://support.apple.com/kb/PH14204

2. Delete Caches.db

Close all windows and quit all applications.

Hold "option" key down and click "Go" menu in the Finder menu bar.

Select "Library" from the dropdown.

Library > Caches > com.apple.Safari > Caches.db

Right click the Caches.db file and select "Move To Trash.

Close windows and relaunch Safari.

3. Empty Caches

Safari > Preference > Advanced

Checkmark the box for "Show Develop menu in menu bar".

Develop menu will appear in the Safari menu bar.

Click Develop and select "Empty Caches" from the dropdown.

4. 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.

5. Safari > Preferences > Extensions

Turn off Extensions if any and launch Safari again to test.

Turn on those one by one and test.