Off topic though, but zPanel is out dated and their team has moved into Sentora..

If it is a new installation, try the sentora

Like what ‘hunky’ said, zPanel is out of date, you sound try following this tutorial to install zPanel’s successor, Sentora: https://www.digitalocean.com/community/tutorials/how-to-install-the-sentora-web-hosting-control-panel-on-ubuntu-14-04

Anyways, are you sure you typed the correct credentials? Almost always it’s a simple typing mistake. I’d reinstall and if it still doesn’t work then there might be a problem.