It may happen that your smartphone loses the internet access overnight, after being switched off, without any modification made to the network settings.
You will notice, however, that some applications are still able to download data, while other won't synchronize.
Reset network settings
You can always restore the default network settings of your phone!
Please note that your personal data won't be deleted during this manipulation:
From the HomeScreen, go to "Menu" -> Settings -> Wireless & networks -> Mobile networks -> Access Points ->"Menu" -> Restore default settings.
It is possible that at this stage the network still does not work ... So read the following.
Set the date/time
To communicate properly, your smartphone and servers will need to provide information about the current time and date (for example, check whether your request has expired). This is particularly necessary for app synchronization.
From the Homescreen, go to "Settings" > "More" > "Date and Time"
Check "Automatic date and Time" to make use of the network-provided time.