Formal:
* Я всё забыл(а). (Ya vsё zabyl(a).) - This is the most straightforward and literal translation. It's suitable for formal situations.
Informal:
* Я всё забыл(а) как в тумане. (Ya vsё zabyl(a) kak v tumane.) - This means "I've forgotten everything like in a fog," implying a complete lack of memory.
* У меня всё из головы вылетело. (U menya vsё iz golovy vyletelo.) - This means "Everything flew out of my head," implying a sudden loss of memory.
* Я ни черта не помню. (Ya ni cherta ne pomnyu.) - This is very informal and means "I don't remember anything at all." It's considered quite rude to use this expression in formal settings.
The best option for you depends on the context and your relationship with the person you're speaking to.