this is good if you just want to drop in some css and get a dropdown navigation back using the default wordpress nav

example of the default styles

code (css)

/* Menu Dropdown */

.nav ul {
  list-style: none;
  padding: 0;
  margin: 0;
  background: #1bc2a2;
}

.nav ul li {
  display: block;
  position: relative;
  float: left;
  background: #1bc2a2;
}

.nav li ul { display: none; }

.nav ul li a {
  display: block;
  padding: 1em;
  text-decoration: none;
  white-space: nowrap;
  color: #fff;
}

.nav ul li a:hover { background: #2c3e50; }

.nav li:hover > ul {
  display: block;
  position: absolute;
}

.nav li:hover li { float: none; }

.nav li:hover a { background: #1bc2a2; }

.nav li:hover li a:hover { background: #2c3e50; }

.main-navigation li ul li { border-top: 0; }

.nav ul ul ul {
  left: 100%;
  top: 0;
}