You will have to do this - I'm just cutting and pasting from my post on the Unity forums. There was an issue in older versions of tk2d, where reference to the font is broken. This usually happens during an upgrade (we narrowed it down to Unity trying to compile the font file as a c# script, and have worked around this, but if its already broken it will need to be fixed by hand).
1. Open up the sprite collection (TK2DROOT/tk2dUI_demo/Assets/SpriteCollection/UIDemoSpriteCollection, and if it isn't this skin, check TK2DROOT/tk2dUI_demo/Assets/Dark/UIDemoDark).
2. Under the "Fonts" category on the left bar, you should see "Empty" there - this indicates that Unity has somehow lost a reference to the font. Click on it (there may be more than one depending on the collection you opened).
3. Note that the BM Font slot will be Empty. This will need to be filled up again - the easy way to find it is, click "Font Texture". The thing that should go into the BM Font slot should be right next to the texture in the project window. Drag that into the slot.
4. Repeat for the other broken fonts. not necessary in this case, as UIDemoSpriteCollection only has one font
5. Commit.
That's it - the references need to be dragged back in again, commit, and once thats done the text should all fix itself.