WhatsApp not opens after WP 8.1 GDR 1

Same issue here Lumia 920 but I get stuck looking the background. After reinstalling it works now.
 
Had exact same issue on 520... Deleted app, reinstalled... works fine now, had even backed up all my chats. :)
 
My whatsapp is broken,I doubt wasn't to uninstall and lose messages.

You actually won't be losing messages. WhatsApp created a backup every night at around 3:30AM. So, if you reinstall, you can restore almost all messages.
 
Man, if only I knew this was a problem with the update. My phone has been acting dodgy for a few wks and after the update I thought that was one of the issues. I was tired of it all so I just did a hard reset and have lost all my convo, now i see I may not have had to do that.

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Reinstall.

yup, reinstalling fixes the issue from hanging up on the whatsapp 'splash screen'.

don't worry about your Groups and old message history: they all get backed up and all the Groups (and settings too) re-appear after it finds your auto-saved backups, upon re-installing.