When trying to restart Qmail are you getting the failure message ?
Check for the mailogs to see if you can spot something. If you are on a plesk server, you can find the logs at /usr/local/psa/var/log/maillog
Do you find an error like :
==================
Date host qmail: xxxxx alert: cannot start: unable to open mutex
Date host qmail: xxxxx alert: cannot start: unable to open mutex
==================
This can happen when you manually try to delete the qmail queue and you might just happen to delete a file from /var/qmail/queue folder. To solve this error, do the following :
# touch /var/qmail/queue/lock/sendmutex
# chown qmails:qmail /var/qmail/queue/lock/sendmutex
Once this is done, restart qmail as
# /etc/init.d/qmail start