what can i do ??
Are you talking about avast Home Edition?
If so, simply get a new one by re-registering.
I you’re talking about the Professional version, just send an email to sales (at) avast.com and explain the problem. Please, use the same email address that you have received the registered key.
When i forgot my key(Home version), in Avast registration page there is an option. You write your mail and voilà, they send your key.