Are you able to send emails but not receive?

Either your domain is over quota, or your e-mail address itself is over quota. You will need to increase the amount of disk space to receive mail again.

Please sign into cPanel and look in the left menu for Disk Space Available. Make sure this is a positive number.

Next, please, click the Email Accounts icon. Check the column that says Usage, and make sure it is not higher than the Quota. You can simply increase your Quota.

How can the email go over quota? You may be leaving your messages on the email server too long (help files back on the main support portal page) and they are accruing to a size too large.