codice:
...
<head>
<script>
   function valida() {
       document.getElementById('btt_submit').disabled = 'disabled';
       
       if (document.getElementById('prova').value == "") {
           alert('campo vuoto');
           document.getElementById('btt_submit').disabled = false;
           return false;
       }
    
    return true;    
   }
</script>
</head>
<body>
<form action="page.php" onsubmit="return valida()" >
  <input type="text" id="prova" />
  <input type="submit" id="btt_submit" value="invia form" />
</form>
</body>

Ciao