I'm sorry to hear you are having this problem. This is the first we have heard of this freeze. I'm going to recommend a reset of the app to try to solve this issue. To do that, open the side menu and press and hold on your name at the top of the menu. Then tap "Reset POS". When you do this you will have to log into your shop again, so make sure you have your email and password for your Shopify store.
If this doesn't solve the freezing, give our support a call at 1-888-678-9481and they will be happy to help out more.