DIAGNOSIS The internal backup battery that maintains the clock and settings is dead. It's a standard 3V lithium coin cell (typically a CR2032) soldered to the main control board.
FIX Open the base of the machine by removing the screws under the rubber feet. Locate the main circuit board. You'll see a small, silver coin cell battery holder (sometimes it's just soldered directly). If it's in a holder, pop the old one out and put in a new CR2032. If it's soldered, you'll need to desolder the old battery and solder in a new one, minding the polarity (+ side up).
IF THAT DIDN'T WORK If the clock still resets with a fresh battery, the problem is on the board itself. A diode or capacitor in the power supply circuit for the memory/clock chip has likely failed. This requires component-level board repair.
HEADS UP When you have it open, check for any signs of water damage or corrosion on the board near where the tubes pass. A previous slow leak could have killed the battery and damaged the board.