Why is my network not working

There could be a number of different reasons.

Have you tried a hard restart? If not, try that first. If that does not work we can try resetting the Network Settings.

Hard Restart:

Simultaneously press and hold the Power button and Volume Down button (right edge) until the Samsung Galaxy screen appears then release both buttons.

Allow up to 15 seconds for the Samsung Galaxy screen to appear.

Try again (assuming your issue is with wi-fi networks?).