Without knowing what happened before you lost your subnav, I’ll walk you through troubleshooting.
First, look at your code.
I viewed your code with Developer Tools (in most browesers press F12) and found your submenu is there. Which means it is likely a missing javascript file or CSS issue.
Retrace your steps.
Is there a change to your theme?
I looked at your CSS from your theme ( http://www.apcm.ca/wp-content/themes/apcm/css/style.css?ver=4.6.1 ) and found the code to hide subnav and reveal when the parent li is hovered over.
SO,
I went back to Developer Tools and navigated down to a ul class="subnav"and unckecked the different values for subnav
then I saw your submenu popped up when top: 100% was unchecked.
THEREFORE
That code is your culprit.
Recommendation: this is not a WordPress issue, but a CSS issue. Post this on stackoverflow for a response.