I figured it out. 1.) went to 'Blocks Admin.' and selected 'Sections' as the show visible blocks setting. (Sections is the only page that the block would appear in).
2.) I now had 'Main Menu' as an option, and I clicked 'Edit' under the 'Action' column. That page presented me w/ a dropdown list of pages that the block would show up in...I chose 'All pages'.
Problem solved; but I can promise you that I did NOT get in the settings that deep and change it prior to this problem....which scares me. Hopefully this is just a fluke. At least I know how to fix it should it happen again.
Thanks again for your time...

