Pro Tools could not initialize the current playback engine

[barbara.goddy]

Hello,

I am using ProTools 9 (LE) and the Mbox Pro 3. The computer specifications are Mac OS X, 10.6.8. Normally I didn't have any problems with launching ProTools but yesterday I saw this message on the screen:
ProTools could not initialize the current playback engine. Please make sure that device has been configured correctly.

I already tried the fallowing two tips, but it didn't help:

[B]1. Launch Pro Tools
When the splash screen appears on the screen, hold down the "n" key on your keyboard, which will eventually open the Playback Engine dialog for Pro Tools. (WHEN I PRESS DOWN THE N KEY, NOTHING HAPPENS! WHEN I CHOOSE THE SETUP MENU, THE LETTERS ARE GREY, SO THAT I CAN NOT CHOOSE THE PLAYBACK ENGINE.)

Mac Only
If the solution above fails, you can try deleting the "com.apple.audio.AggregateDevices.plist" preference on Mac OS X. You can find this preference file in the following location:
Macintosh HD > Library > Preferences I DO NOT HAVE THAT FILE IN MY PREFERENCES!

Thank You very much for the help!

Regards,
Barbara