To investigate why you might be unable to send or receive emails, take these steps:

1. Check that your internet connection is working.
2. Check that your email service or application is running properly.
3. Check that any email filters, rules, or redirects are not blocking the email.
4. Check that the email was sent correctly and reached the recipient’s mailbox.
5. Verify that email storage limits have not been reached.
6. Check your ‘junk mail’ or ‘spam’ folder.