Lets start with the obvious.
- Make sure that the phone is not on Airplane mode. If it is, it will not receive any form of signal from either the mobile signal, Bluetooth, NFC, and WiFi.
- Turn off the device. Leave it alone for one minute. Turn it back on.
Now with the more in-depth steps.
- Make sure that the phone is not on the Lost/Stolen list (though that would only affect the cell phone and data, not WiFi or other methods of connection).
- Recently installed Apps? The apps that claim to be able to clean your phone for free, 9/10 are harmful and can cause problems with the device. Sometimes, gaming apps or wallpaper apps can also bring problems, such as adds and interfere with the device’s use. If you have any apps of this kind, uninstall them, restart the phone and see if that helps.
- Go to Settings, Backup and Reset, Reset signal and other settings. (This will clear all WiFi information, as well as Bluetooth devices previously connected.)
- Last option: Go to Settings, Backup and Reset, Factory Reset. This of course, will reset the phone completely and will erase everything. As this is the most drastic measure, only use it as a last resort.