Q: El Capitan Wifi Problem - Suddenly

Does this issue happen with any Wi-Fi that you connect your MacBook Pro to or just to this particular wireless router? When connected, is it limited to just the local network or can it still gain Internet access?

Ok, then it pretty much identifies that the MacBook Pro Retina is the culprit.

Not sure, at this point, on where to guide you next since I don't know exactly what you have already done to try to resolve it.

I typically offer the following so pick and choose which you haven't tried and please post back your results:

Try the following on your Mac in order, until (hopefully) resolved:

1. Create a New User Account
   • System Preferences > Accounts
   • Click "+" to add a new account.
   • Log off, and then, log back on with the new user account.
   • Try connecting to a Wi-Fi network with your Mac's AirPort.
   • If this resolves the problem, there may be something amiss with your current profile and will require further troubleshooting.
   • If this does not resolve the problem, go ahead and log back into your normal account and go on to the next step.
2. Delete AirPort Keychain Entries
   • Launch the "Keychain Access" application located in Applications/Utilities.
   • In the windows on the left side: Select login for Keychains and "Passwords" for Category.
   • Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
   • Close Keychain Access.
3. Delete Preferred Network(s)
   • System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
   • Delete all entries under "Preferred Networks."
   • Click on OK.
4. Add Preferred Network(s)
   • System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
   • Add the preferred network(s) using the "+" button.
   • Click on OK.
   • Restart or log out, and then, back in.
     • If it works, then you are done.
   • If not, go on to the next step.
5. Move System Configuration Files

  Step 1

• In Finder, go to /Macintosh HD/Library/Preferences/SystemConfiguration
• Move (not copy) the preferences.plist file to the OS X desktop.
• Restart the computer.
• Verify if you have connectivity. If not, go to the next step.

  Step 2

• In Finder, go to /Macintosh HD/Library/Preferences/SystemConfiguration
• Move (not copy) the NetworkInterfaces.plist & com.apple.airport.preferences.plist files to the OS X desktop.
• Restart the computer.
• Verify that you have connectivity. If not , go to the next step.

  Step 3

• In Finder, go to /Macintosh HD/Library/Preferences/
• Move (not copy) the entire SystemConfiguration folder to the OS X desktop.
• Restart the computer.
• Verify that you have connectivity.