You are missing a final closing brace in the headline
definition:
\documentclass{beamer}
\usetheme{CambridgeUS}
\makeatletter
\setbeamertemplate{headline}{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=2.75ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{section in head/foot}\insertshorttitle\hspace*{2ex}
\end{beamercolorbox}}% <- this closing brace was missing
}
\setbeamertemplate{footline}{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{title in head/foot}\insertshortauthor
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
\usebeamerfont{title in head/foot}\insertshortinstitute
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\insertframenumber{} / \inserttotalframenumber\hspace*{2ex}
\end{beamercolorbox}}%
\vskip0pt%
}
\makeatother
\author{The Author}
\title{The Title}
\institute{The Institute}
\begin{document}
\begin{frame}
test
\end{frame}
\end{document}

I took the liberty to slightly increase the height for the box in the headline (this is just a suggestion, of course, and you can revert to your original settings).
To suppress the headline
and footline
, use the plain
option for frame
; to suppress one template only, redefine it locally to be empty:
\documentclass{beamer}
\usetheme{CambridgeUS}
\makeatletter
\setbeamertemplate{headline}{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=2.75ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{section in head/foot}\insertshorttitle\hspace*{2ex}
\end{beamercolorbox}}%
}
\setbeamertemplate{footline}{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{title in head/foot}\insertshortauthor
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
\usebeamerfont{title in head/foot}\insertshortinstitute
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\insertframenumber{} / \inserttotalframenumber\hspace*{2ex}
\end{beamercolorbox}}%
\vskip0pt%
}
\makeatother
\author{The Author}
\title{The Title}
\institute{The Institute}
\begin{document}
\begin{frame}[plain]
test
\end{frame}
\begingroup
\setbeamertemplate{headline}{}
\begin{frame}
test
\end{frame}
\endgroup
\begingroup
\setbeamertemplate{footline}{}
\begin{frame}
test
\end{frame}
\endgroup
\begin{frame}
test
\end{frame}
\end{document}

I think, your footline disappeared, because it was overwritten with \setbeamertemplate{footline}[page number]
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\setcounter{subsection}{1}
\usetheme{Berkeley}
\usecolortheme{seahorse}
\usefonttheme{serif}
\useinnertheme{rounded}
%%Make desired footline
\makeatletter
\setbeamertemplate{footline}
{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{author in head/foot}\insertshortauthor~~\beamer@ifempty{\insertshortinstitute}{}{(\insertshortinstitute)}
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{subtitle in head/foot}%
\usebeamerfont{title in head/foot}\insertsubtitle
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\insertframenumber/\inserttotalframenumber\hspace*{2ex}
\end{beamercolorbox}}%
\vskip0pt%
}
\defbeamertemplate{footline}{page number appendix}
{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{author in head/foot}\insertshortauthor~~\beamer@ifempty{\insertshortinstitute}{}{(\insertshortinstitute)}
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{subtitle in head/foot}%
\usebeamerfont{title in head/foot}\insertsubtitle
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\Roman{framenumber}\hspace*{2ex}
\end{beamercolorbox}}%
\vskip0pt%
}
\g@addto@macro\appendix{%
\setbeamertemplate{footline}[page number appendix]%
}
\makeatother
\usepackage{appendixnumberbeamer}
%%No Navigation template
\beamertemplatenavigationsymbolsempty
%%Title
\title{My beamer slides}
\author{Just another guy}
\institute[ABC]{My famous ABC institute}
\date{worldwide, \today}
%\logo{\includegraphics[width=1.5cm]{./logo}}
\begin{document}
\section{First Section}
\begin{frame}{First slide}
First slide (1/2)
\end{frame}
\begin{frame}{Second slide}
Second slide (2/2)
\end{frame}
\appendix
\section{Appendix}
\begin{frame}
First appednix (I)
\end{frame}
\begin{frame}
First appednix (II)
\end{frame}
\end{document}

Best Answer
Here's one possibility; I redefined the title page template and used TikZ's
\node
s to place the elements;pdfrender
was used for the author and the title:CTAN lion drawing by Duane Bibby.
Feel free to adjust the lengths according to your needs.