If your device's Wi-Fi is turned off, you won't be able to connect to any Wi-Fi network. Double-check if the Wi-Fi is enabled and once it is enabled try connecting to a network.
See the don't know how to enable the Wi-Fi guide should you have any difficulties in enabling it.
By restarting your device, you will kill all the processes running in the background as well as the one which might have prevented you from connecting to Wi-Fi network(s). Also, you will reset the Wi-Fi card (or adapter), which might have failed to load properly.
Maybe the real reason for this issue is that you are trying to connect to another Wi-Fi network which has a similar SSID (name) as your own. Double-check the SSID of your Wi-Fi network and compare it with the one you see on your device and make sure that they are identical.
Updating the operating system is a must, in order to improve the mobile device's functionality and eliminate bugs in the operating systems. Usually, the updates are being applied automatically, but sometimes this is not the case.
See the don't know how to update my device's operating system guide if you are not sure how to check for updates manually.
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