Sounds like either the KPRX file is gone corrupt or you need to run repair tool. Have you tired opening the kprx on a different computer.I have found that sometime the K2 tool are not available in Visual Studio randomly and once you run the blackpearl repair t tool it brings them all back. If your kprx is the problem and you dont have any back up, K2 does store all versions of the workflow everytime it is deployed. You can access and download the kprx from the workspace in the management tab under the process name.