Note:
This is an attempt to create a canonical question that covers all instances of "not-working trackpad" issues.
If you are experiencing a not-working touchpad, but none of the following answers work for you, please do ask a new question and then update the answers for this canonical question when your new question gets answered.
What’s the best way to troubleshoot a non-functional trackpad?
I ask this because almost every day someone asks about a touchpad that isn't working ... I think it would really help if Ask Ubuntu had a systematic generic strategy for how to troubleshoot not-working trackpads.
Ideally, the answer will cover simple things to try first (e.g., keyboard function keys, settings menu) and go on to increasingly advanced methods – ending with what information to post on this site to help people trying to answer if it's unsuccessful.