There apparently were two .Net Security Patches made available last night in WIndows Update. They are causing some problems with DNN, not sure which versions specifically, but I've seen issues reported in 4.3.6 and 4.3.5 so far.
Sebastian posted this in a forum earlier today as fix for the Text/HTML module no longer working. (found in 4.3.6, not sure about 4.4 or later releases)
in /controls/TextEditor.ascx line 9 replace id = ”celTextEditor” Runat=”Server” with id = "celTextEditor" Runat="Server"
The issue being the CURLY quotes. I had another client with the same problem, they had curly quotes in their skin file instead of normal "" quotes, with windows update last night the skin immediately broke.
So if you're having some issues with your DNN sites, start looking for funky quotes and replacing them with standard quotes!