I have been working in AutoCAD 2011 ans saving various copies of my work using "save as". None of the files will open and all come up with the message "drawing file is not valid".
I have tried typing 'recover' into the command line and also inserting / importing the file into a new one but it tells me the files are no good. I have also tried opening the file in other programmes such as Microstation but to no avail.
Does anyone have any ideas as to how I can open the files or extract the data from them?