Ok, after a few days, I've run into to this problem again 3 times. And I managed to resolve it again. But I've noticed something peculiar.
It took the first 2 times to notice something. Everytime, at least in my system, that the icon disappears after reboot, I can't open Avast from desktop. If i try to open it in safe mode, it takes ages and the status is Unknow and this doesn't fix anything.
So, how I fix it. I've installed a little cleanup tool named Bleachbit. It has some hidden options in the edit -> preferences -> don't show irrelevant... something. I'm translating from portuguese, because the program doesn't let me choose any other language (if it's installed in a pc with a different language, it'll install in english, french, etc etc etc).
After that, basically I've selected all options to clean .temp files from all programs on the list. Only .temp files.
I've noticed then that a 30mb .temp file was created by avast. I've deleted this file and rebooted. The Icon showed again and avast was up and runnin'.
The 3rd time it happened I repeated this process. The temp file was only created after a reboot in safe mode and trying to open avast from there. It took about 4 minutes for avast to load into "Unknown Status" and then I rebooted, cleaned the temp file and rebooted again.
This is starting to get annoying. Hope it'll be fixed soon.