Hi. We're other users here, not Evernote. Note History will have details of your previous content, if it was synced to the server for at least 6 hours before loss. Evernote says backups of note content are taken 'several times per day', so it's not guaranteed.
If you notice an unwanted change on any device, the standard 'undo' should work at the time. Otherwise changes are saved to temporary local storage and are probably un recoverable.
AFAIK templates cannot be applied to an existing note. The process if to create a new note, apply a template to create the formatting and standard content, then add your own information as necessary. If you highlighted an existing note and then applied a template, it may have simply deleted the current content. If so, Note History is your only possibility.
You can try contacting Support, but they were overwhelmed before the end of year holiday period and it may take several days to receive their reply. Use https://help.evernote.com/hc/en-us/requests/new?guest=true - and the issue type "Account"