You can also say 'mondai nai'　　’ もんだいない ' :)

大丈夫　（だいじょうぶ）　daijyoubu

We can do the English-Japanese translation "no" is ない and "problem" is 問題 in Japanese No problem = 問題はない

Daijoubu (des)!

It can also be 「大したことではない」, or even something along the lines of 「どういたしまして」. It's hard to answer this exactly without having the context; "no problem" is a rather broad expression you know.