Its much simpler to search for your issue before posting. The answer is in the sticky above, the title should give you a clue.
Given that the sticky says
firstly check internet connection and other on-demand providers - if they all work other than bt then you could try the following
try resetting your box as it may fix it - the 251 code usually means authenication has failed, which could be corrupted data on your box or because someone in bt vision may have changed something at their end.
remember the reset is a last resort as it can affect other paid on-demand products you may use