Is there an error message?
Does PS open other files OK?
It sounds like you have the most current version of ACR, is everything else up to date?
Go to Help|Updates and make sure.
If that doesn't help try resetting your preferences file.
It cures a lot of strange behavior in PS.
Start Photoshop and immediately hold down Ctrl+Alt+Shift (Windows)
or Command+Option+Shift (Mac OS X).
Then, click Yes to the message, "Delete the Adobe Photoshop Settings file?"