glEnable(GL_DEPTH_TEST); weird problem

OpenGL OpenGL: Basic Coding
1

when i render a quad with the following code without
enabling depthtest it shows up normally. but when i enable depthtest it just doesnt show up. what could the problem be?

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glLoadIdentity();
	//glEnable(GL_DEPTH_TEST);
	glColor4f(1.0f, 0.0f, 1.0f,1.0f);

        glPushMatrix();
	glTranslatef( 0, 0, -1 );
	glNormal3f(0.0f, 1.0f, 0.0f );
	glBegin(GL_QUADS);
	glVertex3f( -10, 10, 0 );
	glVertex3f( -10, -10, 0 );
	glVertex3f( 10, -10, 0 );
	glVertex3f( 10, 10, 0);
	glEnd();
	glPopMatrix();  

        SwapBuffers(hDC);

Init :

    	PIXELFORMATDESCRIPTOR pfd = { 0 };
	pfd.nSize		=	sizeof(PIXELFORMATDESCRIPTOR);
	pfd.nVersion	=	1;
	pfd.iPixelType	=	PFD_TYPE_RGBA;
	pfd.cColorBits	=	mBpp;
	pfd.cDepthBits	=	16;
	pfd.dwFlags		=	PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | 
						PFD_DOUBLEBUFFER;
	pfd.iLayerType  =	PFD_MAIN_PLANE;