Workflow Designer has lost the connection to SharePoint Integration. This causes the SharePoint events to no longer be present as the Designer does not recognize that SharePoint has been integrated.
There are missing items from SharePoint. Existing SharePoint events will likely break and/or become unusable.
To resolve this issue and have the SharePoint events return to the Workflow Designer, have your Global Tenant Admin (your highest level permission user) run the registration wizard via your SharePoint's App Catalog. Running the registration wizard will reinitialize the SharePoint integration returning the SharePoint events to the Workflow Designer.