Sorry to hear that. Would you mind providing a screenshot of what you're experiencing? (Here's more info on how to take and post a screenshot from a mobile device: How to add a screenshot to a forum message from iPad/iPhone)
I have a couple questions that will aid me in solving this issue. What version of Sketch are you using? Which iOS are you using as well as what device model? Was there an error message or did Sketch simply stop working?
Hopefully we will have you back up and running very soon. Thanks for your patience.