K. Weird. Theres no point in monetizing if you're not gonna collect that money from the ads. So, to do that you either sign up for adsense or join a network. You choose adsense, cool. When you make your adsense account with all your correct information you have to wait for them to approve your adsense account. After thats done you have to if i'm not mistaking when you go to youtube visit your dashboard and go to channel, theres monetization. Thats where you go and see whats up with it, maybe you accidently disabled it but usually you would go there now and youtube would ask you to authorize or attach i forgot the term but they ask you to add your adsense account to your youtube channel. Then again you have to wait for them to approve or something. When thats done thats when you can go to any of your videos info/settings and go to monetization tab and monetize it (although i think they do that automatically but if not you can go do it manually).
I'm personally with a network right now and if i remember correctly thats how you had to do it if you're with adsense.