Was this a POP account you set up? Very important to review all the default settings, as I think POP accounts default to deleting from server after download. That's common for all email clients, not just Thunderbird. Don't uninstall Thunderbird, as that could cause a loss. Assuming yours is a POP account, you could create an IMAP account of the same email account alongside the POP account, and then copy messages from the POP account to the IMAP account.