Seeing as you're Android, perhaps your SD card has run out of space. I'd recommend deleting some items from the card if this is the case. If it's not however, it may just be a problem with the app. In that case, I'd recommend reinstalling the app, as Google Play's account progress for Pixel Gun 3D is saved via a cloud, so you can reinstall and keep the progress you've made earlier. (I assume that's correct anyways, as I'm only an iOS user).