Adding an additional homepage breaks wordpress site

Hi all,

I am a web developer for a company and I have been handed out a project to develop a completely new and fresh 2.0 website. The company already had a website that was done with the help of wordpress previously by the old developer, who is not with the company anymore.
Before the launch of the new website, I was told to put a countdown page on the old wordpress website (which is installed on the root directory of the website).
Now my problem is that when I change the name of the old index.php (to home.php)file for wordpress and link to it through my new countdown page, wordpress completely breaks down.

Not having any experience with wordpress, is there any setting inside the admin panel that I could change?

Thank you all in advance,

S.