On the video that raised the question I had clicked a link provided on a forum (using Tapatalk) I was on at the time.
The youtube icon wasn't up after I left it, so I'm assuming it opened it from Safari if that makes sense.
Try in this order and move to the next step if it continues to not work:
- restart
- reset (hold power and home button)
- reset all settings (settings > general > reset)
- check for software updates
- backup and restore as new
- contact Apple for replacement