Open the database exclusively, to do this open from file menu, select file
and use the little arrow next to 'open' then choose exclusive from the list.
When it opens select 'Tools', 'Security', 'Unset Database Password'. It will
ask for the password and then remove it. This is presuming it was a database
password that you set that prevents the file being opened without it. If it
is more specific user-level security please give more detail.