As of April 20th, 2015 YouTube discontinued the support of the YouTube API V2 that my ACF YouTube field was using.
For detailed and most up to date directions follow these by Google to obtain an API key. The main thing to note is that you want to use the server key setting.
Here is the quick look, what to do with your API Key is at the bottom:
1. Go to console.developers.google.com and create your project. After you add a name, click on your project in the list.
2. Under API’s and Auth in the side navigation click on Credentials.
3. Click on ‘Create new key’
4. Click on ‘Server Key’ and it will give you a place for your server IP. Server IP address is optional.
5. Your new API Key should appear in the list view on that page. Highlight it and copy it.
6. In your WordPress admin navigate to ‘Settings’ -> ‘ACF YouTube’.
7. Paste your API Key into the ‘YouTube API Key’ setting text box and click save.
8. Back in the console.developers.google.com click into APIs under ‘APIs & auth’
9. In the list of Popular APIs or in the search box find ‘YouTube Data API’
10. On the next page click the ‘Enable API’ button.
This should get you all set to go. On that page you can check your usage and quotas to see how much you are using.