if it is a Horsebox and tacho exempt, the Digi tacho has not been calibrated and the internal battery failed.
When they calibrate a digi tacho they will change the internal battery.
If that is the case, you will need a new tachohead, as they cannot be revived, it looses all his parameters.
the Digi tacho in a truck controls your limiter etc, and its prohobided to use your vehicle in that state.
Its cheaper to keep you tacho in calibration and updated, a new tachohead is approx £500 + Vat and Labour, a calibration you can have for a £100 approx