Power Macintosh 6500: Freeze at Startup

I have a Power Macintosh 5500/6500 or 20th Anniversary Macintosh running either Mac OS 7.6.1 or Mac OS 8. Recently, I was troubleshooting some of my Extensions using the Extensions Manager. Now when I startup my computer, the mouse freezes at the "Mac OS Starting Up..." screen.

This happens before any of my Extensions start to load along the bottom of the screen. What is causing this problem?
Power Macintosh 5500 and 6500 computers can have an internal GeoPort modem card installed in the communications slot. The 20th Anniversary Macintosh ships with an External GeoPort modem, however, this computer can accept a CS II comm slot card in the on board communication slot for Ethernet or modem connectivity.

The Geoport modem card depends on a Shared Library in the Extensions folder called "Serial (Built-in)". If this Shared Library is disabled using the Extensions Manager or removed from the Extensions folder, users may experience a system freeze at startup before any extensions appear to load.

To avoid this problem, users restart with Extensions OFF (by holding down the shift key at startup) and make sure this Shared Library is present in the Extensions folder. If the library cannot be found or has been deleted, users can reinstall this software from the system software CD by custom installing Open Transport from the Networking & Connectivity section.
Published Date: Feb 18, 2012