The nowtv username and password are not linked to your Sky Store account anymore, so if you have made any changes to your nowtv password or Sky Store password you will need to make changes to both accounts if you use the same details.
See this information below which was taking from the Sky Store Help Centre on how to retrieve your Sky Store account details.
I forgot my Sky iD username and/or password. How can I sign into Sky Store?
Click 'Sign in' at skystore.com and you'll see links to 'Forgotten your username or password?' Click and follow the instructions to reset them.