Make sure the account you're trying to import from is still active.
Remove and then re-add the email address you're importing from. Here's how:
On your computer, open Gmail.
At the top right, click Settings Settings then View all settings.
Click the Accounts tab and enter or Accounts.
Under "Check mail from another account," delete the email address you're importing from, then add it back.
If the above steps don't work, check to see if the program Gmail uses to receive messages from other accounts has problems.