My sketchbook pro app froze

Double tap Home.

Tap and hold on one of the icons at bootm.

Tap the minus sign at top left of the Sketchbook Pro app.

Tap Home then tap the Sketchbook Pro icon to launch it again.

I would also check to see if you have recently downloaded new drivers for your pen/tablet. I ran into an issue with the latest Wacom Drivers but I rolled back to February's drivers and have had no issues since. Check for updates from the mac app store as well.