Can you report for which airport you saw U-1 for? It is likely that if you zoomed out far enough it would switch you over to a WAC chart. You can get back by tapping the "Switch Maps" button on the lower right and selecting Sectional. Or simply zoom back in some. We are working on revising these WAC, SEC, TAC zoom transitions...
There is a possibility that some ADS-B or Weather data congestion caused a brief pause... How long did you let it stay frozen for.. I'm wondering if it would have caught up and proceeded fine. If you can zip and send us the "#log" folder within the User Folder, along with the brief "Frozen Pause and reboot resolved..." then we can dig into this a bit more. Just email it to support@adventurepilot.com and we'll watch for it.