Every ferret owner has had periods where their furbaby won't or can't eat solid food. That's where the "duck soup" protocol comes in.
There are many different recipes for this concoction, and there are even some pre-made. The trick is to find one that your ferret will eat.
Recipes can be found at any ferret knowledgeable message board. There's a particularly good board at The Holistic Ferret. A Lot of the information in my articles comes from them. Give them a look!
Anyway, back to the duck soup. It's actually not made from duck; and it's not really soup. It has the consistency of jelly, most times, but can be thinned if necessary to allow for syringe feeding for those times your ferret is simply too sick to eat on his own.
It is a combination of liquid nutrients, much like Ensure for humans. Necessary nutrients in an easy to swallow/digest form. And it is a really good idea to have some on hand, in case your fuzzy becomes ill, has surgery, etc. when feeding is going to become difficult.
Much more information is available at The Holistic Ferret.