\documentclass{report}
\usepackage{titlesec}
\titleformat{\chapter}[hang]
{\normalfont\normalsize\bfseries\centering}{\chaptertitlename\ \thechapter:}{2mm}{}
\begin{document}
\chapter{The big black bug bit the big black bear, but the big black bear bit the big black bug back!}
\end{document}
I would like to have the second line of the chapter title to be in the center of the page.
\documentclass{report}
\usepackage{showframe}
\usepackage{titlesec}
\titleformat{\chapter}[hang]
{\normalfont\normalsize\bfseries\centering}{\chaptertitlename\
\thechapter:}{2mm}{}
\newlength{\chapterindent}
\newcommand{\gethalfwidth}{\settowidth{\chapterindent}{\bfseries\chaptertitlename\
\thechapter:}%
\addtolength{\chapterindent}{2mm}
}
\begin{document}
\newcommand{\breakline}{\\\gethalfwidth\hspace{-\chapterindent}\ignorespaces}
\chapter{The big black bug bit the big
black bear, but the big\breakline
bear bit the big black bug back!}
\setcounter{chapter}{20}
\chapter{The fox, and the duck, and another small animal,
but\breakline also a big red elephant are walking down the road
to\breakline happiness}
\end{document}
The smart way: Calm down and take a deep breath, read posts and provided links attentively, try to understand and ask if necessary.