To troubleshoot this issue, please follow these steps:

Ensure that your internet connection is stable. Try accessing other websites or applications to confirm that your internet is working properly.

Sometimes, emails are mistakenly marked as spam. Check your Junk or Spam folder in Outlook to ensure that the email hasn't been filtered incorrectly.

Go to “Home” in the Outlook toolbar. Click on “Junk” and then select “Junk Email Options.” Check the settings and adjust the filter level if necessary. Ensure that your contacts or safe senders are listed to prevent their emails from being marked as junk.
For more details, please refer Outlook's Junk Email Filter Option.

Check if there are any rules set up in Outlook that could be diverting emails to specific folders. Go to “File” > “Manage Rules & Alerts” to review and modify rules.
For more details, please refer Outlook's Guide for Editing Rules

Ask the sender to confirm that the email was sent successfully. Sometimes, delays can occur on the sender's side or emails might get stuck in their outbox.

Temporarily disable any third-party antivirus or security software that might be interfering with the download of your emails. If you can receive emails after disabling it, consider adjusting its settings.

Ensure that your mailbox isn't full. If your mailbox has reached its quota, you may not receive new emails. Delete unnecessary emails or archive them to free up space.

Make sure you are using the latest version of Microsoft Outlook. Outdated software can sometimes cause compatibility issues. Check for updates and install them if available.