Apple mail doesn't show mail content when selected?

Question

I have done nothing new or strange with this application but since a few days ago I couldn't see mail content when I selected the emails.
I can see the new emails It doesn't show their content when I select them. It simply says no message selected.
Also when I double click to open the mail on a new page the application crashes. Any ideas? This is so frustrating!
Btw, I'm using a Macbook Pro.

Answer 1

Quit Mail. Force quit if necessary.
Back up all data. That means you know you can restore the Mail database, no matter what happens.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData
Copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
          Go ▹ Go to Folder
from the menu bar. Paste into the box that opens by pressing command-V, then press return.
A folder window will open. Inside it there should be files with names as follows:
          Envelope Index
          ExternalUpdates.storedata
Move those files to the Desktop, leaving the window open. Other files in the folder may have longer names that begin as above. Move those files, if any, to the Trash.
Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK. Typically, the process takes a few minutes, but it may take hours if you have gigantic mailboxes. In that case, you may be able to speed things up by temporarily adding your home folder to the Privacy list in the Spotlight preference pane. Remove it when Mail has finished importing.
Test. If Mail now works as expected, you can delete the files you moved to the Desktop. Otherwise, post your results.