<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<input type="checkbox" checked>
<select name="" id="">
<option></option>
<option>1 опц</option>
<option>2 опц</option>
<option>3 опц</option>
</select>
<script>
var sel = document.querySelector('select');
document.querySelector('input').onchange = function(){
sel.disabled = !this.checked;
if (!this.checked) sel.selectedIndex = 0;
};
</script>
</body>
</html>