javascript not refilling after going back

It would all depend on how you handle your state.  Ideally this is done server side via sessions. What are you using server side?
To just suggest an answer based on this one example will not work if the Javascript isn't specific to the page. Eg, when the page loads you could test to see if the first drop down has a selected value and call the same function that is called with the Onchange event. But this could cause issues if you re use the Javascript code on other pages. Very hard to say without seeing how and what you've done to code this

That's right, exactly what I said

when the page loads you could test to see if the first drop down has a selected value and call the same function that is called with the Onchange event.

However, what works for one page may not work with the next.

I don't want the onchange event to run again, because it just loads 10 items in a select box.
I need to know which item was selected.

I tried putting this code

 <script>
      $( document ).ready(function() {
if ($('#make').val != "") {
$('#model').val(<%=session("usermodel")%>);
}
});
</script>

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I set that session on the second page.
When you click back though the page doesn't populate again , it doesn't fully reload, so it doesn't recognize that session variable.

Is there any way to get what the selected index would have been on that second box?

Val us a function not a property so you'll need the brackets (). Also missing were the inverted commas around the session variable to indicate a string and not a variable

<script>
      $( document ).ready(function() {
if ($('#make').val() != "") {
$('#model').val('<%=session("usermodel")%>');
}
});
</script>

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That's what I've argued on post 39747925...

You argue to re-execute the function. It is much simpler, just trigger the change event on the select