"Locked for editing" message

I began working on a spreadsheet via Office 365 last night, returned to it this AM and got a message which reads 'this document is locked for editing' -so new/additional information could not be saved unless I saved the sheet under a new name.

 

This is NOT a feature which I have enabled and neither is it the first time since it has occurred, can anyone tell me how to disable this function - it's a royal pain to have to save each edit as a new document/sheet.