Type in Admin as the account name and leave the password field empty.
If that works, go to file options and reselect the "Log In Using" check box for the file.
Another possibility on Mac systems is that you were in fact using a password protected file, but the KeyChain utility was entering the account name and password for you. Since you are now on a different machine, you wouldn't have Keychain set up to enter these credentials and you'll get a password dialog where you did not see one previously.
Unfortunately there is an initial dialog with the name of the administrator-account (my name that I was using in the other computer) and the password blank (in fact that is correct because I've chosen to leave it blank). So when I click on the OK button there is a message saying that 'the account and the password inserted cannot be used to open the file'.
It seems that because I changed computer the file does not recognize me us the original administrator. It is a very important file and I cannot believe that I cannot open it.
You need to use the same account name as you did previously. When this dialog first appears, FileMaker enters the USER name into the ACCOUNT name box in an attempt to save you the trouble of entering an account name. If the new computer has a different user name specified, you will need to delete this user name from the account name box and enter the name that used to appear there automatically on your original computer.
AND IT WORKED! I was making the error to accept the account name that the new computer automatically choose. This was my whole name and surname, but in the other computer it was only my name.
Thank you very very much!