If the thermostat has been wiped and has no screen display then the EPROM will need reprogramming as this is a digital stat... The only real solution to this is throw it in the bin and buy a new one...
If the clock has just lost its settings, then all you need to do is re-program it... Instructions can be found here:
http://www.gasaid.co.uk/?type=pdf&path=/Manuals/Thermostats/Room/RiteTemp/8050/Operation/
GasAid.co.uk, December 2006