Endl - Undeclared identifier.

I have this code, where I use endl to go to a newline. Pretty simple, usually I don't get such an error but suddenly my project has begun to give my an error on every line that i use endl.

Code:

` cout << "Mathematical Manager:\n";`

cout << "---------------------\n\n";

cout << "Select problem type: " << endl;

cout << "1. Rectangular Triangles" << endl;

cout << "2. Circle Equation" << endl;

cout << "3. Quadratic Equation" << endl;

Here's the part of my code where I first got the error(title). Then I tried to change the endl, to \n (as you see in the first two lines), that helped.

What could be wrong? I would like to be able to use endl, because that's what I've always used. :)

Thx in advance.