I just have copied of Lyx files to one computer to another and i am facing above subjected problem. Complete log is given below.
This is pdfTeX, Version 3.1415926-2.4-1.40.13 (MiKTeX 2.9) (preloaded format=pdflatex 2012.12.19) 8 JAN 2013 21:37
entering extended mode
**chapter-1.tex
...
LaTeX Info: Redefining \ref on input line 87.
LaTeX Info: Redefining \pageref on input line 87.
LaTeX Info: Redefining \nameref on input line 87.
(C:\Users\Mani\AppData\Local\Temp\lyx_tmpdir.p11848\lyx_tmpbuf2\chapter-1.out)
(C:\Users\Mani\AppData\Local\Temp\lyx_tmpdir.p11848\lyx_tmpbuf2\chapter-1.out)
\@outlinefile=\write3
! Undefined control sequence.
\@begindocumenthook ...ox {\autoref {##1}}}\addto
\extrasenglish { \renewcom...
l.87 \begin{document}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Undefined control sequence.
\@begindocumenthook ...#1}}}\addto \extrasenglish
{ \renewcommand {\equation...
l.87 \begin{document}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
...
{}button to indent by 4 spaces which marks up a code section but that exceeds the site posting length – David Carlisle Jan 08 '13 at 21:22! Undefined control sequence.\@begindocumenthook ...#1}}}\addto\extrasenglishmeans that you have commands assumingbabelbut you have not loaded babel add\usepackage[english]{babel}– David Carlisle Jan 08 '13 at 21:23babelvia the LyX GUI: Document --> Settings --> Language: Set Lanuguage pack. – Torbjørn T. Jan 08 '13 at 21:25