Google supports all current browsers and previous releases of browsers. For Internet Explorer specifically, this includes support for IE9 and IE10, but not IE8. If IE9/10 is run with compatibility mode for IE8, users may also be experiencing issues loading Google Calendar.
The fix is easy: Users can enforce the native state of IE9/10 by pressing Shift + F12 and selecting proper Browser mode and Document mode. This will force IE9/10 into the native mode and correctly display your calendar.
You can find more information about supported browsers at http://www.google.com/support/a/bin/answer.py?answer=33864.