I've just upgraded to TortoiseHG 2.0 (which has a brand new GUI) on a Windows 7 x64 machine.
When I right-click a modified file and select Hg Commit . . . from the context menu, I get a "No files found for this operation" error message popping up. The selected file has the correct, "modified" icon, and if select the file from within the new Workbench, I am able to successfully commit it.
Any suggestions for fixing the problem with Committing from the right-click context menu?
Try doing a Hg Status on the whole directory, or even a Hg Update.
If the commit still don't work, I think you have found a bug in TortoiseHG and you should report it to the dev team.