Not uncommon, but after Microsoft pushed out the most recent maintenance update for CRM 2016 Online (v184.108.40.2069) we noticed some ribbon button display issues on some browsers.
Microsoft Dynamics CRM 2016 Online – Service Update 5
You may notice missing ribbon buttons (SAVE, SAVE & CLOSE etc) on forms/records
A simple clear cache on your browser should get you back up and running.
Post-clearing cache ribbon buttons return.