Did you renumber the loco (reconfigure the decoder)? If so, try the default number or '3'. Sometimes the Hornby decoders default back to this after a short.
If it doesn't work, it could be that the decoder is 'fried'. In that case, you'll have to take it back to the shop for a replacement.
If it doesn't work, it could be that the decoder is 'fried'. In that case, you'll have to take it back to the shop for a replacement.