Xcode 5 can't open .m file

Xcode 5 can't open .m file

I installed Xcode 5 a few hours ago. I opened my project with it. I have
several .h and .m file in my project. I could view all of them except one
of my .m file. When i clicked that .m file, the cursor became rainbow and
nothing happened.
To be sure , i checked my project folder, go to the that .m file and click
get info. I could see the preview that file. Also i can open it in text
editors.
I couldn't find the problem. Any advice is appreciated.
Best Regards,
Taha