Are you using SharePoint Designer 2010 workflows in SharePoint Online? If yes, you can check below:
Since 3/7/2019, SharePoint Designer 2010 workflows stopped working on SharePoint Online sites, this issue is logged at Office 365 Admin Center > Health > Service Health > Advisories > SharePoint Online. Currently, it has not been fixed. If you meet this issue in your thread, you can suggest customers to check service health and wait for update.
After the above, you also should check the issue in Service health frequently, once this issue is fixed, please update it to your customers in your thread.
If you are using SharePoint Server, please tell us.