I must have done something wrong. I have a C# project in Visual Studio 2008. All of a sudden I see a lock on my classes and when I hover the class names on the top tab I see the class name as : C:\Myprojects\Oder.cs[Read Only] !
Has something weird like that happened to you before?
In my case it was a ReSharper 9.0/9.1 related issue. After clearing the ReSharper cache, I was able to edit files in my solution again.
Clearing the ReSharper cache can be done here: ReSharper -> Options -> Environment -> General
and click: Clear Caches and restart your visual studio.
(The image is contained within the first of the linked JetBrains articles).
JetBrains instructions: