How to check why reboot command failed and machine was instead stopped?

I issued a reboot command but the server failed to reboot instead we lost communication with it, and we had to call the company to reset it.

I am not an advanced users, but I want to check the log messages to see why failed rebooting properly. It is an ubuntu linux.

Please kindly tell me where to look and how to read the log files.

Linux Log files and usage

=> /var/log/messages : General log messages

=> /var/log/boot : System boot log

=> /var/log/debug : Debugging log messages

=> /var/log/auth.log : User login and authentication logs

=> /var/log/daemon.log : Running services such as squid, ntpd and others log message to this file

=> /var/log/dmesg : Linux kernel ring buffer log

=> /var/log/dpkg.log : All binary package log includes package installation and other information

=> /var/log/faillog : User failed login log file

=> /var/log/kern.log : Kernel log file

=> /var/log/lpr.log : Printer log file

=> /var/log/mail.* : All mail server message log files

=> /var/log/mysql.* : MySQL server log file

=> /var/log/user.log : All userlevel logs

=> /var/log/xorg.0.log : X.org log file

=> /var/log/apache2/* : Apache web server log files directory

=> /var/log/lighttpd/* : Lighttpd web server log files directory

=> /var/log/fsck/* : fsck command log

=> /var/log/apport.log : Application crash report / log file

To view log files at shell prompt

Use tail, more, less and grep command. tail -f /var/log/apport.log\

more /var/log/xorg.0.log

cat /var/log/mysql.err

less /var/log/messages

grep -i fail /var/log/boot