If you are using ESET/NOD32 security/AV then probability is very high it is causing your problems. According to their forums, you should temporarily disable SSL until they solve the problem per https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products?ref=esf - the problem is being reported by dozens of users https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/ https://forum.eset.com/discover/

I followed the suggested procedure which involved disabling and then enabling SSL filtering and then restarting the computer. It worked as Thunderbird now downloaded my new emails. Thank you Wayne for your help.