A password protected excel file can sometimes be recovered using specific tools such as HashCat or John the Ripper. These are quite technical password "cracking" programs.
Older versions of office (prior to office 2010) had quite weak password hashing algorithms but the newer versions are much harder to recover. Tools such as those mentioned above can attempt to guess the password very quickly using long lists of possible passwords.
If the file does not contain confidential information you could try an online password retrieval site that uses the same techniques as the aforementioned tools.