The process of building custom solutions for SharePoint Server.
A couple of days ago I faced the same issue. The SharePoint 2013 Workflow opened blank while newly created workflows worked fine (2010 + 2013). I found the SharePoint Designer Version to be the root cause. It was not the Service Pack 1 that was missing. Instead it was a difference in patch level - something after SP1.
The vanilla installation from https://www.microsoft.com/en-us/download/details.aspx?id=35491 gave me version 15.0.4420.1017.
After running Windows Update / Microsoft Update and a bunch of Office 2013 Updates later SharePoint Designer had version 15.0.4849.1000.
I then started SharePoint Designer and after restart again I could see my workflow actions again and problem was solved. So long story short: Check your SharePoint Designer patch level! A workflow built with a newer version of SharePoint Designer will not open in older versions.