Microphone gone from my keyboard?

Go to settings, Language and input, manage input methods and activate Google Voice Typing as an input method and it will show up again

If there's no any voice input method. That means you should install a relevant voice typing keyboard. Like Google Voice Typing from the store.

Rarely but sometimes keyboards can have a native support for speech recognition. Identify the app that you're currently using for alphanumerical typing and access its settings from language and input settings and try to see whether there's an option for activating voice typing. If it is there good, if it's not, the previous is your only option