After DOM is loaded, document.write
replaces all content on page. Instead you probably want to have a element container on body and display messages in that.
<input id="clickMe" type="button" value="Search for Another Part">
<div id="messages-here"></div>
<script type="text/javascript">
function search_part () {
var part = prompt("Stackoverflow example: ");
if (typeof part === 'undefined') {
alert("That part is not in the database.");
} else {
document.getElementById('messages-here').innerHTML = part;
}
}
window.onload = search_part();
document.getElementById("clickMe").onclick = search_part;
</script>