My phone's 3g/4g randomly stopped working I have a Galaxy S3 and my 3g/4g suddenly stopped working. WiFi works fine, but how can I get my 3g to work again? I have already uninstalled apps and deleted unnecessary photos and videos.
Check that flight mode is not swithed on. This mode will disable any network (Wi-Fi, cellular network, etc).

Else, go to Menu | Settings | More Settings | Mobile Networks | Network Operators (Note: your menu options might be different due to the Android version installed on your S3) and reconnect to your telecommunication provider's cellular network. Note: You may need to select your telco provider's name when your S3 displayed a list of cellular providers.

Also, shutdown your S3. Then, take the battery out for 30 minutes. Reinsert the battery. Boot up your S3 and turn on Mobile Network option and see if 3G/4G has been restored.

Worst case, it could be an Android firmware's update that is causing the S3 to fail to connect to any cellular network.