Hope to get a solution for this problem soon as I have the same problem.
Can you look up the Event Viewer if you find any information for the time when it happened?
Nothing showed in the Event Viewer log. However, just to be thorough:
1. I uninstalled reader 6.0.1 and rebooted. (all pdf became unassociated)
2. I deleted any acr*.tmp anywhere on my computer.
3. I deleted The 6.0 folder from the application updates path
4. I deleted the 6.0 folder from my install path.
5. I deleted the Adobe folder from my temp directory.
6. I ran CCleaner, cleaned app files and registry and repeated.
7. I rebooted and repeated step 6.
8. I opened Event Viewer, backed up and cleared all events from the log.
9. I installed 8.1.2 (all pdf became associated with Acrobat Reader)
10. Under Event Viewer/Application the event log displays: Product: Adobe Reader 8.1.2 -- Installation operation completed successfully.
11. Under Event Viewer/System the event log displays: The Windows Installer service entered the stopped state.
12. I tried to launch: the shortcut, a pdf and the exe file.
13. There are no additional events added to any level of the event log.
14. There is no indication of activity from Reader in the Task Manager: Applications or Processes. It never appears in either.
As a final note, I know the 8.1.2 install file is not corrupted because I copied it to another computer where it installed and runs just fine.
We noticed the same thing but our problem is this.
Our user can launch Reader just fine, but when they exit out of Reader and then come back to launch it later, it won't launch.
We discovered this, if you open up the Windows Task Manager, you'll see the AcroRd32.exe running. If you kill this task, then you'll be able to launch it again. But unfortunately, we see this as a re-occuring problem.