Best bet is probably to do the above, trash preferences etc (the usual) and failing that ask on the AVID Pro Tools support forums as they're a more technical forum and less about sound design where as this is the opposite. They'll be able to get you sorted for definite i'm sure :)

http://duc.avid.com/forumdisplay.php?s=f8813aed26a07f99959ff66a38d6cab6&f=84

Have you recently installed OS updates of any sort? Given how PT has to be basically re-written to fix for each new Mac OS major release, I wouldn't be surprised if a hotfix got installed and it tripped it up (maybe some hotfix Avid advises to avoid), especially if one day after working fine it's stopped indefinitely and older versions don't work either.

Where does it stop loading it (e.g. a certain plugin, the DAE, etc)? Did it kick back some sort of system error message? If so, what did it say specifically? Did PT crash, freeze, did the OS kernel panic, or did PT just sort of stop responding? Are any and all lights on the hardware function properly to indicate that your device is being detected by the system? Is your firmware up to date? What version of 9 are running, and what version of 8 were you trying (each minor version of PT has it's own quarks in terms of compatibilities with the OS and hardware firmware)? Have you checked the Avid docs for these particular versions of 8 and 9 top see if there was anything special to take note of (one such case I believe being that during one of the minor 8.0.x updates, it required all Digi unit firmware updated to a 9.x firmware)?

Unfortunately to be any more specific, further details would be greatly appreciated.