@HelenP Yes Crowd is a Public Forum and not a direct support point for Optus.
Have you checked for reported outages? Use the above link and click on Fixed Outages (All NBN connections are Fixed Connections). If you use 4G/5G click on Mobile Outages.
If you connected to the NBN you can also check https://www.nbnco.com.au/support/network-status
you need to enter your full address.
If there are no reported outages you use social media to contact Optus Support, please send a PM to Optus on Facebook or a DM on Twitter and include in the message that messaging is not working from the My Optus App.
@HelenP Web Chat was discontinued a few months ago. Messaging is the replacement.
Have you tried logging into your Optus Account via a web browser and sending a message?
There is an icon at the bottom right corner of the page. I cannot think of anything else to suggest.