Compare your settings with the recommended ones here:

https://www.centurylink.com/home/help/internet/email/how-to-set-up-your-email-server-settings-to-go-through-pop-and-smtp.html#server-settings

and state your antivirus and VPN, if in use.

Server SEttngs for Pop:

• POP server name: pop.centurylink.net
• Port: 995
• Connection Security: SSL/TLS
• Authentication Method: Normal Password
• Username : full email address
• Password: same as used to access webmail account

Outgoing Server SMTP:

• Server name: smtp.centurylink.net
• Port 587
• Connection Security: STARTTLS
• Authentication Method: Normal Password
• Username : full email address
• Password: same as used to access webmail account

Checks on computer: Firewall this might be controlled by defaulton computer or by an Anti-virus you installed. Access Firewall and make sure Thunderbird is set up as an allowed program

VPN Servers do not like VPN as it hides your IP address and server may think you are a hacker, so it blocks access. Please check computer for a VPN. Some Anti-Virus products now offer a VPN and it may have been enabled by default. Switch it off or create a slpit tunnelling and set up Thunderbird to use it.