Have you filled in any tax returns? Were you registered self employed? Again was turnover above Vat threshold? I agree you need an accountant who is experience with small business tax investigations. I am afraid whilst being ill is unfortunate as far as HMRC are concerned it is no defence.
No one here could give you impartial advice based on such a small amount of information. There is no short cut - you need to ask a small accountant, or the Money Advisory people, or go to a Citizen's advice bureau.
The legal part with the card needs to be also addressed - but this is for a solicitor.