CAD tip # 7937:

1. Collision with a system component taking care of file access, resp. attached to the Open dialog - e.g. a network redirector, antivirus software (e.g. a known problem with Norton 360 3.0), virtual disks, a thumbnail generator, etc. You can avoid displaying this dialog by temporary disabling of FILEDIA.
2. The Open dialog itself - if AutoCAD seemingly freezes instead of displaying this dialog, it is possible that the dialog is displayed "off-screen". Use the tip 5749 to take it back to the visible part of your screen.

Another reason is just a slow file access (apparent freezing) caused by a heavy load of your PC or network, or the source disk or a network resource is temporarily unavailable, or there is an error on the disk media.