I would like to use \autoref{fig:subfig:a}, but I don't get the figure number by simply typing this. My code is:
\documentclass{article}
\usepackage{amssymb,amstext,amsmath,latexsym,mathtools}
\usepackage{pdfpages}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{float}
\usepackage{hyperref}
\begin{document}
\section{Aya}
\autoref{fig:ABC:a}
\begin{figure}[H]
\centering
\subfloat[this is ...]{
\label{fig:ABC:a}
\includegraphics[width=0.45\linewidth]{ABC1.pdf}}
\hspace{0}
\subfloat[this is ...]{
\label{fig:ABC:b}
\includegraphics[width=0.45\linewidth]{ABC2.pdf}}\\[20pt]
\subfloat[...]{
\label{fig:ABC:c}
\includegraphics[width=0.45\linewidth]{ABC3.pdf}}
\hspace{0}
\subfloat[...]{
\label{fig:ABC:d}
\includegraphics[width=0.45\linewidth]{ABC4.pdf}}
\caption{abcd}
\label{fig:ABC} %% label for entire figure
\end{figure}
\end{document}
What should I add to display the number of the figure correctly?

\subfloatisn't recognized. I gather that this is a macro that's provided by thesubfigpackage -- which, by the way, is not loaded in your example code. Given that thesubfigpackage is quite obsolete, though, you should consider using thesubfigureenvironment that's provided by thesubcaptionpackage. An example of how to use this environment is provided in the answer I've given. – Mico Jun 27 '12 at 16:20