Hi @Anonymous User
Bit of a long shot, if you have a nowtv username (not forum name) try that instead of your email address when you sign into the LG Smart TV NowTV App.
Otherwise maybe delete the NowTV App,then re install the App, power the TV down at the main power outlet for a minute, then see if you can sign back into the App on your TV.
I don't own a LG TV so i won't be able to offer any further help, other to point you in the direction of live chat from the link below.
https://help.nowtv.com/contact-us/managing-my-devices