If that's the case then you could use awk to join the pair of lines and then grep just the date of interest.
$
$ cat f45
01/11/2010:
No of users 100
02/11/2010:
No of users 102
03/11/2010:
No of users 99
$
$ awk '/:$/{printf $0 " "} !/:$/{print}' f45 | grep "02/11/2010"
02/11/2010: No of users 102
$
$
Or you could use Perl -
$
$ perl -lne 'BEGIN {$d="02/11/2010"} if (/$d/){printf("%s ",$_); $x=1} elsif ($x){print; $x=0}' f45
02/11/2010: No of users 102
$
$