It depends why you have been asked to undertake the medical. If it is for Incapacity Benefit, then this is a quite separate benefit from DLA. If you fail it, then your Incapacity benefit will stop (you can, of course, appeal) but your DLA will not be directly affected.
However, if the reasons for your failure relate to your health problems improving (in other words, if you can now do things which at the time you got the DLA you could not do)this could be something you should have reported to the DLA people as a change of circumstances . If you fail to report it & the change is subsequently discovered, you could be faced with an overpayment and demands to repay it.
If your medical has been requested by the DLA people, then the result could be to stop or adjust your DLA (but you can appeal).