Crouching makes my player unable to move.

So I am trying to make my character crouch. However it stops the ability to move all together. Can anyone figure out why.

if (isStanding == true) {
			float forwardSpeed = Input.GetAxis ("Vertical") * movementSpeed;
			float sideSpeed = Input.GetAxis ("Horizontal") * movementSpeed;

			// Determine movement speed in every direction
			
			Vector3 speed = new Vector3 (sideSpeed, verticalVelocity, forwardSpeed);	
			speed = transform.rotation * speed;
			characterController.Move (speed * Time.deltaTime);
		}

		//Crouching
		if (Input.GetKeyDown(KeyCode.C) && characterController.isGrounded)
		{
			// Determine movement speed in every direction

			float forwardSpeed = Input.GetAxis ("Vertical") * crouchingSpeed;
			float sideSpeed = Input.GetAxis ("Horizontal") * crouchingSpeed;

			Vector3 speed = new Vector3 (sideSpeed, verticalVelocity, forwardSpeed);	
			speed = transform.rotation * speed;
			characterController.Move (speed * Time.deltaTime);

			Crouch (forwardSpeed,sideSpeed);
		}

public void Crouch(float forwardSpeed, float sideSpeed) {
		if (!isCrouching) //Crouch
		{
			isCrouching = true;
			isProne = false;
			isStanding = false;
			characterController.height = crouchHeight;

			
			//		characterController.center = Vector3(0, -0.5, 0);
			//		tempCamera.transform.localPosition.y -= crouchHeight;
		}
		else //Stand back up
		{
			isCrouching = false;
			characterController.height = charHeight;
			isStanding = true;
			isProne = false;
			//		characterController.center = Vector3(0, 0, 0);
			//		tempCamera.transform.localPosition.y += crouchHeight;
			
		}
	}

You are only trying to move your character the moment C is pressed, you need an if statement similar to your isStanding check:

if (Input.GetKeyDown(KeyCode.C) && characterController.isGrounded)
{
	Crouch (forwardSpeed,sideSpeed);
}

if (isCrouching == true)
{
	// Determine movement speed in every direction
 
	float forwardSpeed = Input.GetAxis ("Vertical") * crouchingSpeed;
	float sideSpeed = Input.GetAxis ("Horizontal") * crouchingSpeed;
 
	Vector3 speed = new Vector3 (sideSpeed, verticalVelocity, forwardSpeed);    
	speed = transform.rotation * speed;
	characterController.Move (speed * Time.deltaTime);
	
}

although I recommend you just change your movement speed when you crouch and then change it back when you stand back up and reuse the same movement code, instead of one for each movement type.