DOPUS suddenly quit previewing movie files in viewer pane

Do movies play okay in Windows Media Player? It's not exactly the same as Opus but they both rely on the same underlying stuff to play videos so it's a good first test.

The first section of this FAQ, which suggests temporarily assigning filetypes to Windows Media Player to repair registry settings, may help:

[HOW TO: Enable/fix playback of various media formats in Opus)