Mail doesn't start after upgrading to Yosemite

Question

Hi,
I upgraded to Yosemite and Mail wasn't working for a couple of days... I couldn't send any email... and now when I try to open Mail
It seems to start, the Mail menu bar appears on top of the screen but it never opens any window.
I upgraded in 2 mac mini I have and everything worked fine, this problem I have is on my macbook air.

Answer 1

Back up all data before proceeding.
Step 1
If Mail crashes or freezes immediately on launch, try the steps suggested on this page. Sometimes a corrupt message on a mail server can be deleted by logging in to the server through its web page. If Mail still crashes on launch, skip to Step 3.
Step 2
Select all your mailboxes, and then select
          Mailbox ▹ Export Mailbox...
from the Mail menu bar. Export the mailboxes to the Desktop folder.
Make a note of the settings for all your Mail accounts – everything you'd need to reconstruct the settings from scratch.
Quit Mail.
Step 3
In the Finder, hold down the option key and select
          Go ▹ Library
from the menu bar. Move the following items (some may not exist) from the folder that opens to the Desktop:
          Application Support/AddressBook/MailRecents-v4.abcdmr
          Containers/com.apple.corerecents.recentsd
          Containers/com.apple.mail
          Containers/com.apple.MailServiceAgent
          Mail
Note: you are not moving the Mail application. You’re moving a folder named “Mail.”
Launch Mail. It will behave as if you were setting it up for the first time. Go through the setup process with one of your accounts, using the information you noted earlier. Test. If Mail works now, recreate the rest of your settings.
If there’s no improvement, quit Mail and put back the items you moved to the Desktop, replacing any newer ones that may have been created in their place. Stop here and post your results.
Step 4
This step should not be necessary with IMAP or Exchange mailboxes, because they synchronize automatically with the server. Nevertheless, if the mailboxes are very large, importing them may spare you the need for a long download.
If you took Step 2, import the mailboxes you exported:
          File ▹ Import Mailbox...
If you skipped Step 2, look inside the Mail folder on the Desktop for a subfolder Mail/V2/Mailboxes. Import the mailboxes it contains.
Test. If Mail is still working, delete the items you moved to the Desktop in Step 3.