I connect my app to Localtunnel. It was working very well until last week, but now it's not working:
Edit: Localtunnel appears to be working again. (Of course depending when you're reading this it may not be. Please use your own judgment.)
Original answer:
The cloud-hosted part of Localtunnel is currently down. You'll have to wait until its maintainer brings it up again, or use another service like ngrok.
Some more issues tracking this: