Center hrule in the middle of the page

You can replace

\hrule width 0.5\textwidth

with

\rule{0.5\textwidth}{.4pt}

Also, if you don't need the center environment for anything else, you can replace

\begin{center}
\rule{0.5\textwidth}{.4pt}
\end{center}

with simply

\noindent\hfil\rule{0.5\textwidth}{.4pt}\hfil

MWE

\documentclass[10pt,a4paper]{article}
\begin{document}

Hello everyone !

\begin{center}
\rule{0.5\textwidth}{.4pt}
\end{center}

\noindent\hfil\rule{0.5\textwidth}{.4pt}\hfil

\end{document} 

Output