If your UE Boom 2 portable speaker has stopped producing any kind of sound, see the UE Boom 2 Is Not Producing Sound problem page for possible causes and solutions.
If you pull up at the bottom of your iPhone and swipe to the left there should be a "Now playing on (your device)" make sure that's switched over to the speaker.
If that doesn't work try pushing the play/pause button if the speaker while it's connected to the iPhone this has happened to work on multiple cases with Bluetooth speakers/headphones.
If all else fails go into settings and forget the device then restart both your phone and speaker. Reset the speaker by holding the minus button and power on for approx. 10 seconds.