Thanks for reaching out. Monetization can be disabled for several reasons. Here's more about it and what to do if it was an error: https://goo.gl/cNar2n . Hope it helps.
While we can't share specifics, the best place to check is your Monetization page in Creator Studio: https://www.youtube.com/account_monetization …. From there, you’ll see an explanation with links to relevant resources. You’ll also see the option to reapply from that page in 30 days.