Computer hangs when opening an attachment in Emails

I recently received an email with an attachment, and it was not in the junk mail or notified as a spam. But whenever I tried to open the attachment, my computer got stuck and also the system did not support the way of rebooting in the proper way, which caused me to switch off the computer to restart. Why does this happen? The virus guard is also enable and up to date and does not show any significant active threats. What should I do to view the attachment?
