2012 Toyota Prius Owners Manual - Page 806

5-2. Steps to take in an emergency STEP 4 After the hybrid system has cooled down sufficiently, check the engine coolant level and inspect the cooling system for leaks. STEP 5 If necessary, add engine coolant up to the "FULL" line. Water can be used in an emergency measure if engine coolant is unavailable. (→P. 499) Have the vehicle checked at nearest Toyota dealer as soon as possible.