Problem with character controller and collisions

For some reason, after saving a quitting my project then reloading it, my player [complete with CharacterController script] keeps falling through the floor. What happened, how do I fix it and how do I prevent this from happening again?

I know that similar questions have been answered, but I havetn't been able to find one that helps with this problem..

EDIT** I get an error that says "NullReferenceException: Object reference not set to an instance of an object"

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPersonController : MonoBehaviour {
 
     public float moveSpeed = 10.0f;
     public float mouseSensitivity = 5.0f;
     public float jumpSpeed = 7.0f;
 
     public float rotUpDown = 0.0f;
     public float headTiltRange = 60.0f;
 
     float vertVelocity = 0;  
 
     CharacterController charcont;
     // Use this for initialization
     void Start () {
         Screen.lockCursor = true;
         CharacterController charcont = GetComponent<CharacterController>();
     }
     
     // Update is called once per frame
     void Update () {
         //Rotation
         float rotLeftRight = Input.GetAxis ("Mouse X") * mouseSensitivity;
         transform.Rotate (0, rotLeftRight, 0);
 
         rotUpDown -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
         rotUpDown = Mathf.Clamp (rotUpDown, -headTiltRange, headTiltRange);
         Camera.main.transform.localRotation = Quaternion.Euler (rotUpDown, 0, 0);
 
         //Movement
             float fwdSpeed = Input.GetAxis ("Vertical") * moveSpeed;
             float sideSpeed = Input.GetAxis ("Horizontal") * moveSpeed;
 
             //Gravity
             vertVelocity += Physics.gravity.y * Time.deltaTime;
             Vector3 speed = new Vector3 (sideSpeed, vertVelocity, fwdSpeed);
 
             //Jumping
         if (charcont.isGrounded && Input.GetButton ("Jump")) {
                 //vertVelocity = jumpSpeed;
             }
 
             speed = transform.rotation * speed;
 
         charcont.Move (speed * Time.deltaTime);
     }
 }