If it has a sign on system, make sure you have logged on to make sales. Otherwise it could be memory loss - especially if the battery hasn't been changed on time. I know of a manual for it which might help:
https://the-checkout-tech.com/manuals/cashregisters/Towa_FX-400_and_Geller_FX-400_operating_and_programming_manual.html
Registerman, March 2019