if it was an IMAP (protocol) based mail account in TB , then nothing is lost , in IMAP mode all email remains in remote mail-server , only a copy is downloaded into TB = Thunderbird. so just again add another mail account in TB as an IMAP based account , all emails (copy) will be available for your access.

if it was a POP (protocol) based mail account in TB , then emails from remote mail-server are completely downloaded (aka MOVED) first into TB = Thunderbird and then shown to user , So if you've deleted POP (protocol) based mail account from TB , then emails for that POP based mail-account in TB and emails in remote mail-server , both are deleted + lost.

please wait for other supporters to inform you further , if there is a way to recover emails from a deleted POP based mail-account.

http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Recover_Deleted_Account