File `revtex4.cls' not found [closed]

Question

OS: Windows 7 SP1
TeX: MiKTeX 2.9
IDE: Kile 2.1

When trying to compile a latex document I get the following error:
myfn.tex:32:File `revtex4.cls' not found. ...blish.aps.org/revtex4/ for documentation)]

I get this error when I click the PDFLaTeX button. I am very confused as there're 2 papers I am working on right now, and the other one works fine. Can someone point out as to what's the issue?

Edits:
Acting on Werner's directive, I have added the following:
For the document that works the preamble is:

\documentclass[journal]{IEEEtran}


% *** MISC UTILITY PACKAGES ***
\usepackage{graphicx}
\usepackage[caption=false]{subfig}
\usepackage{hyperref}
%\usepackage{caption}

% *** MATH PACKAGES ***
\usepackage[cmex10]{amsmath}
\usepackage{amssymb}
\usepackage[mediumspace,mediumqspace,Gray,squaren]{SIunits}

% Conditional package:
% \usepackage{ifthen}
% \newboolean{LDF} %LDF - Let's Do Figures!
% \setboolean{LDF}{true}
% End of conditional package.

\begin{document}

For the document that gives the aforementioned error, the preamble is:

\documentclass{AIAA}

% *** MISC UTILITY PACKAGES ***
\usepackage{graphicx}
\graphicspath{/fig}
% \usepackage[colorlinks=true,citecolor=blue]{hyperref}
\usepackage{color}
\usepackage[sort&compress]{natbib}
% \usepackage{cm-super}

% *** MATH PACKAGES ***
\usepackage[cmex10]{amsmath}
\usepackage{amssymb}
\usepackage[mediumspace,mediumqspace,squaren]{SIunits}

\begin{document}