How to get back Calender in top bar of Ubuntu 14.04?

Try reseting its configuration
dconf reset -f /com/canonical/indicator/datetime/

If it didn't show up try kill it, it should restart
pkill -f datetime

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For similar cases, better to explain how you may get it:
1.
Check if it is running, if yes then possibly a configuration problem
$ ps ax | grep -i indicator-datetime
2863 ? Ssl 0:00 /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service
4043 pts/0 S+ 0:00 grep -i indicator-datetime

2.
Check for where is its configuration. Look for /etc/ or dconf in /usr/share/glib-2.0/schemas/. List package files using:
$ dpkg -L indicator-datetime
[...]
/etc/xdg/autostart/indicator-datetime.desktop
/usr
/usr/share
/usr/share/glib-2.0
/usr/share/glib-2.0/schemas
/usr/share/glib-2.0/schemas/com.canonical.indicator.datetime.gschema.xml
[...]

3.
Locate its dconf path:
$ grep path /usr/share/glib-2.0/schemas/com.canonical.indicator.datetime.gschema.xml