I recently upgraded from Office 2007 to 2010, and that went fairly smoothly. However, afterwards, my Calibri font was gone, so my Word and Outlook started using the next available font in the list... Comic Sans!

I browsed to Control Panel > Fonts, and saw that there was no longer a Calibri listing.

I know that one way to recover a missing font is to just copy the missing font files from another computer. In my case, the four files I needed were:

calibri.ttf,  //regular 
calibrib.ttf, //bold
calibrii.ttf, //italics
calibriz.ttf, //bold and italics

However, once I attempted to copy-and-paste these files to my computer, I realized these files were already on my problem computer; they never really got erased. I don't know why they aren't working in Office, and they aren't listed in Control Panel > Fonts.