I have forgotten the password for an important file(spreadsheet) How can I open it?
You can’t. You wouldn’t expect anyone being able to open your file without the correct password, would you? If there were a way for you, then anyone could use that way.
Actually it is very easy. Rename the file extension ZIP. So replace ODS with ZIP and then use a program like 7Z to open it as a folder. In the folder, open content.xml with a reader like Notepad++. There is your file. If you want to remove the password,
@gallagheria: The questioner asked about a “file password”. Saving a file with password will create an encrypted file for which on the first level the comment by @mikekaganski holds. On a different level some hints may help to find the correct password again. This was discussed in a thread now archived under the link @ajlittoz posted.
The weak sheet passwords your suggestion would apply to, are something completely different.