You can use the ":edit" command, without specifying a file name, to reload the current file. If you have made modifications to the file, you can use ":edit!" to force the reload of the current file (you will lose your modifications).
For more information, read
:help :edit :help :edit! :help 'confirm'
from - http://vimdoc.sourceforge.net/cgi-bin/vimfaq2html3.pl#5.8