I have experienced this issue several times on different farms. After some manipulations with the workflow using SharePoint Designer or a web browser, occasionally, it started to be impossible to open published workflow. The others workflows continued to work as expected, whereas when I tried to open the one that started to misbehave it showed the following error message “Failed to load the workflow definition for the workflow”.
I tried different things, but nothing helped. It seemed to be an issue inside SharePoint. Several days ago I found the following Hotfix: SharePoint Designer 2013 hotfix package (Spd-x-none.msp): June 11, 2013, which seemed to be the fix for this issue.
Today, I stumbled into that issue again and decided that it is a good time to test, whether this hotfix fixes this issue for me. I installed it and the issue disappeared!
Solution: So, the solution in my case was to install this hotfix for SharePoint Designer 2013.