Since April 2017, YouTube introduced an additional requirement for channels to have their monetization active. Channels would need to have at least 10,000 lifetime views for YouTube to consider running ads in a channel's videos, which would only happen after YouTube reviewed the channel.
However, since then, some channels still remains with their monetization active, due to a bug that YouTube is already fixing. If you have less than 10,000 lifetime views in your channel and your monetization was still active ( icon in your videos) but now it's not, this means that YouTube has fixed the bug in your channel. Check your channel's monetization page for more details. For other known YouTube issues, you can check this page.
To get your monetization back you will have to meet the 10,000 views. If you are a Freedom! partner, make sure to contact our support team, as we may have some tips to help you grow and get those views faster. You just need to open a ticket by clicking in the "Submit a request" link at the top of this page.