Quick time player quit "unexpectedly" and won't reopen when I click the reopen button on the window that pops up when I try to play clips.I'm on an iMac with Lion.

You can move it to the Trash. It will be replaced automatically.

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of this test is to determine whether the problem is localized to your user account. Enable guest logins* and log in as Guest. For instructions, launch the System Preferences application, select Help from the menu bar, and enter “Set up guest users” (without the quotes) in the search box. Don't use the Safari-only "Guest User" login created by "Find My Mac."

While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault in OS X 10.7 or later, then you can’t enable the Guest account. The "Guest User" login created by "Find My Mac" is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

Hold down the shift key while launching the application. Any difference?

This procedure is a diagnostic test. It will make no changes to your system.

Quit the application if it's running.

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the page that opens.

Drag or copy — do not type — the following line into the Terminal window, then press return:

sudo opensnoop -n QuickTime

You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up. You don't need to post the warning. If you don’t have a login password, you’ll need to set one before you can run the command.

Now launch the application. Text will begin to scroll in the Terminal window. When the application crashes, quickly switch back to Terminal and press the key combination command-period to stop the command.

Post the text output — please, not a screenshot — from the Terminal window, below what you entered.

If any personal information, such as your name, appears in the output, edit before posting, but don’t remove the context.

You can then quit Terminal.

Important: There seems to be an obscure bug in OS X that sometimes causes input devices to lag or stutter after running opensnoop. If you notice this behavior, simply reboot to clear it.