I made the Cheese Fondue and it turned into 2/3 rubber cheese and 1/3 milky wine that refused to come together. A previous ASK suggested that more cornflour would help salvage the split fondue but it cooked into the rubber cheese to make it form more of a block.
Nigella's recipe for Cheese Fondue (from NIGELLA EXPRESS uses a combination of gruyere and camembert cheeses. Gruyere gives the fondue flavour and the camembert melts easily to give the fondue a smooth and slightly creamy texture. If a fondue is starting to separate then adding cornflour (cornstarch) mixed with a little cold water usually brings it back together again as the cornflour binds moisture to the cheese solids.
However if the fondue has turned into a solid lump of cheese then it is most likely that it has been overheated. You may also have noticed some fat seeping out from the cheese. Unfortunately it can be difficult to recover the fondue if this has happened but it will help to stir the fondue frequently as it heats up and also to put the pot over a medium heat and remove it from the heat as soon as the pot has come to the boil. If you are transferring the fondue to a heater once it is ready, make sure that the heat is not too high and continue to give the fondue the occasional stir as you eat.