When a well running company becomes sick,what are the remedies available for rescuing the company?

Perform in-depth analysis to clearly identify the causes, build-up a team of specialists on the fields related to the causes and "squeeze" their brains to find the best solutions for the company. Sometimes this can be done with internal resources, otherwise you can use external sources like auditors, business controllers, due-diligence specialists. From my previous experience, internal resources can show sometimes reluctancy to this "business scan" due to the fact that results can reveal internal weakness, or even miss-conduct.