case 24:
            {
            SendClientMessage(playerid,0xFFFFFFFF,"[DEBUG]: Checkpoint ID: 14");
            TogglePlayerControllable(playerid, false);
            if(keys == KEY_SPRINT)
          {
            TogglePlayerControllable(playerid, true);
            return 1;
            }
 }

// On top:
new IsInCheckPoint[MAX_PLAYERS];

// At OnPlayerEnterCheckpoint (case 24)
IsInCheckPoint[playerid] = 1; // 1 will mean in this case the entered checkpoint 24

// OnPlayerKeyStateChange
public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
  if(IsInCheckPoint[playerid] == 1) // check if the player is in checkpoint 24
  {
    if(newkeys & 1024) // check if they pressed ALT
    {
      TogglePlayerControllable(playerid, true); // unfreeze the player
      IsInCheckPoint[playerid] = 0; // set this variable back to 0 so when you press ALT unfreezing wont be toggled again
      return 1;
    }
  }
  return 1;
}