This error means that, although you normally have a clerk signed in, there is no clerk signed in at this time. It sounds like you've solved that by logging in with a clerk number. And that should stay logged in indefinitely unless something happens like a power outage.
The bad news is however that it sounds like your programming is been erased. Since is defaulting back to the thank you come again and it doesn't seem to have your retail items in it, it's possible that it was unplugged while the batteries were removed. This would cause it to lose all its memory. It should always be plugged in and the batteries should always be working so that if one source of power fails the can at least hold it's programming. Does it appear that any programming is in the register? If so then that assessment name may not be accurate. And if that's the case, let me know exactly what you are typing in for example, to save a PLU to the register so we can determine if the key sequence is correct.