You'll need to wait until your ad account is deactivated and your balance is paid before reactivating it. To reactivate your ad account, click the Reactivate link at the top of Ads Manager and follow the steps. You can't reactivate an ad account if it has been deactivated due to a change in currency or time zone.
If your Facebook account was disabled volitionally by you, you can reactivate it by logging in. If your account was disabled by Facebook, however, you'll need to submit an appeal to get your account back; depending on the circumstances of the disabling, they may or may not grant your request.