Password for spreadsheet has suddenly stopped working; just says 'incorrect password'

Password to open .odt spreadsheet file has worked fine for months. Suddenly it is ‘incorrect password’ and c annot open file. Please help me to recover this! Urgent!
R Beaton

Please make sure that CAPS-lock is not set.

If that is an encrypted .ods file there is nothing we can do. Restore your last known good backup.

It is not an encrypted file, it is PASSWORD PROTECTED. The problem occurred when I saved an Excel file which used this password,as an .ods file. When Itried to open that, it prompted me for the password,and it didnot work. (Of colurse, thinking it WOUL work, I deleted the Excel version.

How can i recover the password please?
Thanks

Is it the spreadsheet document protected, or a single sheet protected?