Well to make it work again pretty much depends on why it stopped working. So let's try a few troubleshooting things to see if we can get the mic back.
It could be something simple like a rouge app not releasing control of the mic. To solve this go to your settings>applications and find permissions. In your permissions there should be one for the microphone. Disable all permissions there. Don't worry this is temporary, the next time an app needs the mic it will ask permission again. If this restores your mic, as apps begin asking permission note when permission was given and if the issue occurs again. The one of the last apps given permission is likely the suspect behind your mic issue.
Try starting the device in Safe Mode. This basically has the same effect as the above but it has the added affect that it can prevent other internal processes from blocking mic access. Obviously when you do this you will want to try making a video call or testing the mic some other way.
Do you have sound in personal videos you shoot? This could be a sign that the mic no longer works.
A work around would be a pair of headphones with a built in mic. or even a mic with a 3.5 mm jack.