iostream is a very basic C++ header which should be present everywhere where is C++. Check your include path, it might happen somehow that C++ headers vanished from it. Another possibility is that you are using a very old C++ compiler which comes from times, when there were no "iostream" and similar headers (only iostream.h, iostream.hpp and so on). Search your disk for file "iostream" (no extent). If you find none then consider installing more contemporary C++. Less likely, you might have installed a pure C instead of C++ but in this case, you should not be able to compile a C++ program at all.
