Hello,
You should activate it back .. I will explain in some minutes .. But the reasons why it could have disactivated is that :
You have too many friends (5000 is the limit) and you have been considered as a group
Your account has been reported by someone (or a group of people)
You made too much advertising or made too much spamming on people's walls
well these are among the most common reasons why your account might have got disabled. But if you desactivated yourself, this is how you can reactivate it:
Try to sign in with the same log in name and password, you will have a confirmation email to be able to reactivate it again, but if it has been disabled by the admin, then I'm sorry to say that it might have been the last time you have used your account. You can somehow make use of the same log in name but with a different email account and start it all over again. This is all I can do for the time being!