Screen Freezing in locked mode

Hello,

Ive had this only happen twice in 2 weeks since owning the watch.

I always have the watch screen locked to prevent accidental usage and save battery power.

This morning, I had a timer set for my eggs, after the timer finished, I held down one of the buttons to unlock the screen and stop the timer, the timer stopped, but the screen stayed locked with a black background. the only button that did anything was the back light. The screen was frozen for a good 20 minutes.

Any ideas?

William