Fighting this issue for 3 days now (and I'm an experienced break/fix tech)
You can try system restore, but in order to do that your going to have to completely uninstall avast! and reboot first (make sure you reboot). For whatever reason, even with the SDM turned off, system restore isn't able to modify the definitions folder for avast! and will fail.
My first system restore has brought back Cortana, but I still don't have a start menu.
As far a rebooting to fix..... I'm sorry to say that you are only at the beginning of the problem my friend. It started out like that for me too. Reboot/Logout would fix it every time. Then, a few days later..... start & cortana are both completely unresponsive.
When I figure it out, I'll post