I had an error while downloading OS X Mountain lion from the App Store.
It tells me this message "An error has occurred" and nerver wants to start downloading again.
What should I do now!!!
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Have a look at this question: How do I troubleshoot "An error has occurred" when trying to update the Kindle app in the Mac App Store?
Also Apple has a general troubleshooting flow chart here.
In your case I would sign out of the store, make sure the date on your Mac matches an Internet time server and try the download again from the purchased tab. If that doesn't work, you can seek support from the link above.