In case Blynk app was working for you and then accidentally you can't login anymore and see message like "User is not registered". This is likely will happen in case you are moving abroad. Please follow link in order to resolve your issue :
In case you wasn't using Blynk for a long time (> 6 months) - unused accounts were removed recently during migration to new infrastructure. Sorry for inconveniences. Please create account again and send me your login name in case you are kickstarter backer in order to restore your energy.
Regards, Blynk Team.