The default page is supposed to take you to the thankyou.php page. You will need to have this page on your server in the same directory as the contactus.php page. If the page is trying to send you to the thankyou page then it is working as it should. You should start with this default file and attempt to make your updates here.
There is not a lot going on in these pages but if any part of the php code or js script is changed up it can make the page not work, so you will need to be careful in how you apply your updates making sure to save and test the page regularly to ensure that it is still working.