Odometer lost reading

[johnsie boy]

Can anybody help ? i took the battery off my Lotus Elise S2 2001 to charge it as it had been in storage for a few months and when i put it back on the mileage that had been covered has been lost and the reader went to zero, I drove the car 85 miles took battery off and charged again and mileage done has stayed, does anyone know how to get back lost mileage please . instrument panel is a stackpanel if this helps .
please call me on 07970216605 paula or perry