Bootstrap navbar button toggler suddenly stopped working (hamburger menu)

Question

when I clicked on the hamburger menu it used to work then suddenly stopped

<nav class="navbar bg-dark navbar-expand-lg navbar-dark">
    <a class="navbar-brand" href="">Tindog</a>
     <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav">
        <li class="nav-item">
            <a class="nav-link" href="">Contact</a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="">Pricing</a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="">Download</a>
        </li>
    </ul>
 </div>
</nav>

I've included bootstrap and jquery .