1. Start up from a Mountain Lion Recovery system holding Command-R during startup.
2. Then, choose Utilities > Terminal from the menu bar.
3. When the Terminal opens, simply type resetpassword and then press the Return key.
4. Select the system drive containing the inaccessible primary account you wish to reset.
5. Choose the name of the inaccessible primary account from the pop-up menu.
6. Enter and reenter a new password and optionally a password hint in the appropriate fields.
7. Click the Save button to save the new password.
8. Quit the Reset Password utility to return to the Mac OS X Utilities window.
9. Quit the Mac OS X Utilities window to restart the Mac.
I came across this problem too, and I found the simple answer.
If you are wanting to run the 'resetpassword' routine (even if only to reset not the password but the user permissions / ACLs), then yes you can follow the steps described by Eric Ross above. However, before you get that far, you need to unlock the encrypted drive.
In other words:
-Start up holding command-R.
-Choose Disk Utility.
-Choose the drive you need to decrypt and then click Unlock.
- Now quit Disk Utility and follow Eric's instructions from step 2. (NB - If you are NOT resetting the password, but instead resetting the ACLs, then be careful to instead follow the instructions in the link I have given above.)
Wait - sorry. I think I forgot the OP's central problem, namely a forgotten password. Obviously you need the filevault password (which is the admin password) to unlock the disk. I suspect this means that my tip above is only useful to those who know the password and either want to change it or want to reset ACLs and user permissions etc.
I should add that it is not entirely clear to me what the OP means by "Now my admin password is not recognised." You would think that either it is forgotten or mistyped - or it was mistyped to begin with. But I could be wrong. I hope the OP sorted this out long ago, of course...