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I am guessing you have tried temporarily powering down the box at the mains outlet plug didn't work whilst logging into the NowTV App ?
See if there is anything on this link article below helps with signing into the NowTV App on your BT Youview Box.
Also if you have a NowTV username (not Forum Name) try that instead of your email address.
Apart from that see if live chat can assist & help you further.