The game decides whether you're online by trying to contact a few specific servers and reading their reply. Occasionally the problem is universal and we need to take action, but usually it's specific to the player's particular situation. Here are some things to try:
- Turn your device all the way off (as opposed to putting it to sleep), then turn it on again.
- Confirm that your network doesn't restrict access to certain sites. (E.g., some corporate networks prevent their employees from accessing many external websites.)
- Confirm that your firewall rules aren't overly restrictive.
- If you have ad-blocking software on your device, try temporarily uninstalling it and see if this makes a difference. Note that you have to actually uninstall it; simply disabling it doesn't always have the same effect.
- Confirm that your internet connect isn't slow or intermittent/unreliable.
- Confirm that your device's date and time are set correctly.