Did you upgrade your FME in the time between editings? I recently had a 2014 workspace that, when I opened it in 2015, all I got was an empty canvas. It turned out to be some characters in annotation objects that could not be parsed (not sure the exact reason).
Anyway, as Jonathan says, send it to our support team and we will get the developer to take a look at it. I am sure it can be fixed... as long as you haven't made any manual edits. You definitely don't want to start editing fmw files by hand.
For the record, the upper "half" of an fmw file is XML(ish) that defines the workspace in Workbench; i.e. what objects are where on the canvas and how they are connected, etc.
The lower half of an fmw file is Mapping File code. This is the part used by the FME engine when the workspace is run. Every time you edit the workspace in Workbench, both parts get updated.
So you'll see that it's not easy at all to manually edit the file. If you make changes to the upper half only it won't affect how the workspace is run, and if you make changes to the lower half only then it won't affect how the workspace is depicted - plus the lower part gets entirely regenerated as soon as you open the file in Workbench and make a change.
Basically it's not practical to make manual edits to the file contents. If you ever think your file has been corrupted like this, please do send it to our support team asap and we'll probably be able to figure out how to fix it.