File revtex4.cls' not found [closed]

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{color}
\usepackage[sort&compress]{natbib}
% \usepackage{cm-super}

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

\begin{document}

-
What does the preamble look like... for both documents? Edit your answer with this detail. – Werner May 14 '12 at 19:02
It sounds to me like the paper that works is in a separate directory which has a local copy of the revtex4.cls. The quick fix would be to copy it across to the other directory. The not-so-quick fix is not so quick – cmhughes May 14 '12 at 19:42
The more recent revtex4-1 is shipped with up-to-date versions of MiKTeX and TeX Live. However, revtex4 isn't. You may have to download revtex4 and manually install this. See How can I manually install a package on MiKTeX (Windows) (the installation process should be fairly similar to non-Windows systems, if need be). – Werner May 14 '12 at 20:15
I never heard of a publisher that does not accept revtex4-1 as well. I would go with Werner's comment and simply replace revtex4 with revtex4-1. Werner should make that an answer, it always worked for us. I.e.: \documentclass[options here]{revtex4-1} with the same set of options in the square brackets as it was before. – Andy May 15 '12 at 6:08
at everyone: Thanks a lot for your help! I am not sure how it was solved, but somehow after installing lots of packages (including revtek4.cls) and updating Miktex (using updmap and FNDB from the settings of Miktex) things are looking normal. I wonder if I should close this question as it is not adding any value just sitting there. – Shashank Sawant May 16 '12 at 7:08

closed as too localized by Torbjørn T., Marco Daniel, Frank Mittelbach, Count Zero, egregJun 2 '12 at 22:42

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