What version of Thunderbird? Which operating system? Which email provider? What firewall or security software.

You could post your settings, which would answer most of these questions. In Thunderbird, go to Help|Troubleshooting Information. Clear "include account names". Click "copy text to clipboard", paste into your reply here.

https://support.mozilla.org/en-US/kb/cannot-receive-messages

Let me know how can I share my computer screen with you? That will help the most as I am new to thunderbird.

This forum is the official Thunderbird support venue. There is no phone support. If you think it helps explaining the problem you can add a screenshot to your post. You're supposed to follow instructions and suggestions given above.