The battery will be flat. You will need to wipe the programming and start again. You will find instructions on clearing random access memory near the front of your manual. Perform a RAM clear first, then set the date and time. Now you need to see if the battery has survived after being held flat for so long. To do this, leave the machine turned on in REG mode for a day to charge up. Then turn the power off at the wall for 1 day. If it is still ok when you turn back on (check that the date and time are correct), it is safe to start re-programming it. Run through your manual and program each section (or have a dealer do it). If the memory is lost when you turn back on, you will need to have a dealer open the machine and solder a new battery in to it.
If you need a manual, try C-tech:
http://the-checkout-tech.com/manuals/cashregisters/Samsung_ER-290_operating_programming_manual.html
Registerman
March 2013
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