1. Check your sort settings. If your emails are being sorted by date, check that the email is not older than the date range set in your sort settings.

2. Look in the deleted items folder. It is possible that you have accidentally deleted the email and it has gone into the trash folder.

3. Check if you have any filters set that may be preventing the email from appearing in your inbox. It may be that the email you are looking for is being filtered out and sent to another folder.

4. Check for any server or service outages that may be affecting your email service. This could be preventing emails from being delivered to your inbox.