If you just don't turn up, the DWP will treat you as 'failure to attend a medical' and your benefit could be withdrawn.
If you're not well enough to attend, just 'phone and tell them and another date will be arranged.
If you're 'too poorly' to attend at a Medical Examination Centre (MEC), just get a letter from your GP, and it may be possible to be examined at home.
Hope this helps.
Pat