ICANN would take control and find a new operator. There is a pretty long and tedious process involved with operating a gTLD, including proving that you have the appropriate finances and resources to support the domain.
In addition, ICANN requires registries (the companies that run TLDs) to escrow their data so that it won't be lost in case they go out of business. http://icannwiki.com/Data_Escrow
Also the underlying software managing the registry is often outsourced to companies like NeuStar. I imagine if that is the case, they could keep operating it without issue until a new caretaker is found/organized/whatever.