There can be a few reasons for the Thunderbird Not Receiving Emails issue. The first possible explanation is that the email account was improperly configured to receive messages. 

It's also possible that the email server is experiencing technical difficulties. Contacting the email service provider will allow you to determine whether or not the email server is currently experiencing issues. Last but not least, there is a possibility that emails will not be downloaded from the server if Thunderbird is not configured to do so automatically.

Check Password
First, check if you’re using the correct password by logging into your account via the webmail interface. For example, if you have a Gmail account:

Go to gmail.com.
Type your email address and then your password.
If you’ve input an older password, Google will prompt you to reset it.
You can reset your password by clicking the “Forgot email?” message.

Greetings,
Peter