Hello,
I updated latest version of the program yesterday to 2014.9.0.2021 (Avast Internet Securities, licenced), and since then the firewall is desactivated and wont start whatever i do (resolving issue through avast, desactivating windows firewall and so on)
Any tip?
Thanks in advance !
Try a repair…Control Panel>Uninstall a Program>double click “avast”>click “repair”>reboot
It is working now, thanks ! 
Good to hear. When avast! 2015 final version is released, suggest removing the old 2014 version completely, rebooting and installing the new version, all whilst offline. You should have good to excellent results in performance and reliability if this is done (clean install of avast! 2015). Otherwise, updating within the program within the same version (2014 or 2015) should not be ordinarily an issue like the one you just experienced.
Newer versions (not updates to current version) tend to have many new features and overlaying the new code over the old tends not to work out as well as a clean install, where all the files would be fresh and new and of the same version.
Be sure to save your licence where you can find it later, if you do do a clean install.
Thanks for the tip i will keep that in mind. And yes it was an upgrade of an older version (i think i didnt updated program for one year or so), probably the reason of the problem.
Glad you have running now. 8)