Check your mobile data or WiFi connection to make sure it is working. If you recently changed your email account’s password, it will also need to be updated within your app’s settings. The process of changing your password will vary by phone. For iPhones, check out this forum discussion on Apple’s site. For Androids, click here.
You can also try accessing your email from a computer. Double-check that the login information you are using matches with the credentials you’re using on your phone.
Also, within your email app, make sure that the sync feature is turned on. This is usually located under your email account settings or under “Accounts” in your general “Settings” menu. There should be an option to turn on “Auto Sync Data.”