how to reactivate an user account
 

An user account has expired:
Last password change : Jun 29, 2012
Password expires : Sep 27, 2012
Password inactive : Oct 04, 2012
Account expires : never
Minimum number of days between password change : 7
Maximum number of days between password change : 90
Number of days of warning before password expires : 28

What is the command to reactivate it?
Thanks.

try adduser <username>

Look at section 7.2 here
http://tldp.org/HOWTO/Shadow-Password-HOWTO-7.html
It appears "passwd -u <username>" may unlock account