There are tons of tutorials on how to add an announcement banner using PowerShell or custom web parts, but why go through all that when you can use plain-old CSS to get the job done via the UI?
If you browser to the Master Page section within the Site Settings area, there is an option titled “Alternate CSS URL”. Within the file used here, you can easily add the following CSS to inject a quick announcement to all of your SharePoint pages:
content: "[Enter desired text here]";
Apply your changes and refresh the page to see your announcement on display.
If you find yourself running into this error, here’s one solution that you might not find elsewhere (at least at the time of this writing). Check the SSL Settings > Require SSL checkbox for the SharePoint Web Services site and the SecurityTokenServiceApplication subnode. Make sure that it is unchecked as SharePoint accesses it locally over port 80.
This is an obscure self-own, but if you like to set your IIS configuration via script, it’s easy to accidentally set this and (seemingly) brick your WFE.
If you find yourself configuring IIS to use a non-default folder structure/location (hi DISA STIGs!) you might also find yourself encountering some head scratching HTTP 500.19.5 errors in your IIS logs (aka 500 19 5). Make sure that the account your AppPool is using has read access to the wwwroot folder.