Bitcoin has vanished for 3 weeks? I sent bitcoin with a 0.00000494BTC fee to an online market vendor. The transaction took 4 days to confirm by which time the market had cancelled the order. This was over 3 weeks ago now but no bitcoin has come back into my electrum wallet. Do I have to do anything to get it back? Or will it eventually show up? Or is it gone for good? Any help would be really appreciated.
If you're saying the transaction is confirmed now, then those coins are now in the possession of the market.  You can't reverse the transaction - that is what "confirmed" means.  So in order to get your coins back, you will have to ask the market for a refund.  Nobody else has the power to make it happen.

The fact that the market cancelled the order is purely internal to them.  It didn't prevent the money from being transferred - in fact there is no way they could do that.

It is possible that they will refuse, or that they deleted the private keys corresponding to the destination address before the transaction was confirmed.  In that case, too bad.  You could try to get your money back through legal process (a lawsuit or similar), but you can't do it unilaterally through purely technical means.