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.
How can the email go over quota? The last message may have been large, or the quota may have been decreased while the email account was at its current size.