I need to recover my autocad files. Every time I try to open a .dwg file it says "drawing file is not valid".

My HDD crashed and I restored my data using a recovery utility and since then my autocad files cannot open and giving the above messages.

The file sizes are correct. Could anyone direct me to a tool fixing corrupted files. Or a method to open the existing files. I tried to open the .bak and .sv files but they are having the same problem.