If enabled, Workspaces have frames that maintain groups of files. You may be able to go back to a frame the assembly is still in. I think each frame is created when any item is checked in or checked out.
Files in the local cache are effectively encrypted. I don't think you can even see the part file names. In Windchill there is a wrapper file that seems to include the part/assembly file and other information. This is unlike Intralink (not to be confused with WindChill Intralink) which kept a small directory from which recovery was possible.
If you managed to create the assembly and then remove it from the Workspace without any other Windchill activity then it may be gone. On the other hand, perhaps this is a great time to try to run the trail file to recreate the assembly.