Not a hypothetical. It is the single most common way a shop loses its records.
Everything that had gone up, which on a normal day is everything up to the last few minutes. Sign in on any other Android phone with the same number and the shop is there — items, prices, bills, khata balances. What is at risk is only what was made during an outage and had not been sent yet, which is why the queue count is on the home screen rather than hidden in a settings page.
Whatever is dry. There is no second copy anywhere, and this is the one failure paper has no answer to at all — a book is a single point of failure that is also flammable, soakable and losable. Most shopkeepers who have lost one describe the same thing: not the sales, the udhaar.
Usually nothing, and this is worse than paper because it felt safer. An app that stores everything locally and asks for no phone number has nothing to restore from and nobody to ask. Check this before you trust any billing app with a year: if it never asked you to sign in, it cannot give your records back.
The columns above describe categories, not products, and that is deliberate. A claim about somebody else's app is true on the day it is written and stops being true the week they ship something — so a page with a rival's name on it ages into a false statement that we then have to be told about. What is described instead is a pattern of behaviour you can check against whatever you are actually using, in about a minute, without taking our word for any of it.
No card and nothing to cancel. Install it, sign in with your phone number, and the shop is yours from the first bill.