Q: MacBook stops connecting to WiFi

HI there PJ Temple,

It sounds like you are trying to connect to the same Wi-Fi network that used to work fine for you but when you try it tells you that the network could not be joined. I would start by restarting both your Router and modem. Then boot to safe mode. This will clear some system level caches and perform a disk check and repair if needed:

Try safe mode if your Mac doesn‘t finish starting up

Follow these steps to start up into safe mode.

1. Start or restart your Mac. 
2. Immediately after you hear the startup sound, press and hold the Shift key.
3. Release the Shift key when you see the Apple logo appear on the screen.

After the Apple logo appears, it might take longer than usual to reach the login screen or your desktop. This is because your Mac performs a directory check of your startup disk as part of safe mode.

To leave safe mode, restart your computer without pressing any keys during startup.

If you don't hear a startup chime after pressing the power key, see what to do if your Mac won't turn on.

Then leave safe boot once you log in and try joining the network again. Please let us know how that goes! Thank you for using Apple Support Communities. 

Thank you for the update PJ! Next I would reset your PRAM and SMC in that order then test out the network connection again:

How to Reset NVRAM on your Mac

&

Resetting the System Management Controller (SMC) on your Mac

If the issue persist, can you please test this behavior in a new Admin user and tell me how that works out?

1. Choose Apple menu > System Preferences, then click Users & Groups.

2. Click the lock icon  to unlock it, then enter an administrator name and password.

3. Click the Add button  below the list of users.

4. Click the New Account pop-up menu, then choose a type of user.

    
    
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     Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.

      

5. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.

6. Enter a password for the user, then enter it again to verify. Using a password hint is recommended to help the user remember his or her password.

7. Click Create User.

From: OS X El Capitan: Set up users on your Mac

Looking forward to hearing back from you! 

Alrighty, good info. Next I would reinstall the Mac OS software from your recovery partition. Your data should not be compromised but it is always a good idea to have a backup with Time Machine before you take this step. It will require an external hard drive of some sort, and here is some additional information about that:

Use Time Machine to back up or restore your Mac

Then use this article to help you reinstall the OS:

How to reinstall OS X on your Mac

Then test it out again. If the issue persists then the next step would be to erase the hard drive and install the OS again and test it with nothing restored. For that use the section of the previous article titled "Erase your drive and install OS X".

If it works then I would next restore your backup from Time Machine and then test it again. It may very well be the case that your OS just needs to be wiped and reinstalled but thats usually not the case.

I know the process looks tedious but this should help us get to a resolution one way or another.