This morning, my Stremium app disappeared. This evening, I discovered that several of my apps will not stay open for more than 2-3 minutes. Netflix and Prime Video have repeatedly crashed after every attempt. The internet connection is excellent. There are no new updates available. This is a Roku Smart TV, not a USB-powered Express with similar issues.
A quick search makes me think Stremium is pretty shady and that's probably why it disappeared, but that shouldn't have anything to do with your other problem. Try the simple things first - unplug your TV for several seconds and then plug it back in.