If you try to record a new video and only see black, you may have denied the Friendclips app permission to use your iPhone's or iPad's cameras. To fix this:

Go to the Settings app on your device
Tap Privacy, then tap Camera
Make sure the switch next to Friendclips is green or to the right

If the app won’t let you record a new video at all and instead asks you to upgrade, you have hit the free limits of the app. Upgrade to Friendclips Unlimited in order to add as many videos as you want.

In either case, you can always record the video using the Camera app on your device, and add it into Friendclips later.