I found it twice. First in a song:
And then the title of a manga:
じゃない alone is for negation (no happy end, no playing). What I can't understand is the の at the end of the sentence... what does it stand for?
の(だ)/のです with a rising tone are the abbreviations of の(だ)か/のですか。
(The combination だか isn't really used in everyday life to the best of my knowledge).
They turn the sentence into a question which, combined with じゃない make it a tag question, as you can see in the links I posted in my comments.
遊びじゃない？ 遊びじゃないか。 遊びじゃないの（か）？
It's a game isn't it? It's a game right? You're playing aren't you?
Without the rising tone (or か), じゃない is used to deny what precedes it. の/のだ/のです have many different meanings (look it up on this site), the most common ones being adding an explanatory and/or exclamative tone to the sentence.
It's not a game. It' not a game! (What the hell are you doing, ) it's not a game! (I can't believe you take it so lightly, ) it's not a game (you know?).
The sentences in の are often used to answer the implicit question "What is happening?" : "Well, what's happening is..."
誰かが来た？ (Gathering information after coming home).
誰かが来たの？ (Seeing 2 cups of coffee on the table).
(What happened,) someone came?
I don't feel comfortable enough with those grammar to go further into details, I would advise you to look for informations about the differences between だ/です VS の（だ）/のです and （だ）か/ですか VS の(だ)か/のですか.
EDIT : Just to be clear, here are the different forms :
Simple Abbreviated Using の Abbreviated using の
猫だ。 猫。 猫なのだ。 猫なの。
猫（だ）か。 猫だ？/猫？ 猫なの（だ）か。 猫なのだ？/猫なの？
猫ですか。 猫なのですか。 猫なのです？