no problem vs now then

no problem

• no thanks is necessary; you're welcome (said in response to an expression of gratitude) 

• no apology is necessary; think nothing of it (said in response to an expression of regret) 

• certainly, sure (said when granting a request) 

• Something easy, not difficult, not posing problems. 

now then

• A greeting. 

• Used to move or return to another subject of discussion. 

• A mild admonishment.