After doing the upgrades for my forum, it crashed. I don't see the template anymore and even the admin CP is weird with no layout.

What to do? It's urgent!

I maded already a support ticket about this..

Re upload the FULL version of IPB 4.0.5.1 from the client area - then try open your forum

But I think I lost then the members and messages too, no?

Nope, just replace files and go to: http://yourwebsite.com/admin/upgrade

But I think I lost then the members and messages too, no?

​Depends of what you did. If you do not have a backup ask your hosting company to restore a backup. Normally they make regular backups.

Welcome to IPB v4...!

I maded already a support ticket about this..

​Then sit... and wait. You'll have an expert taking a look rather than fumbling about and potentially causing more damage. Patience, in this instance, is very much a virtue.