I've had this problem before.  I'm not exactly sure what causes it, but rarely I'll open a dataset in the design view and only the XML markup shows.  The first time this happened I made the mistake of running some Visual Studio reinstall command I found out there and lost a bunch of my personal settings.  Here's the quick, painless solution:

1. Close the dataset file. 
2. Find the file in the solution explorer, and click on the plus sign [+] to expand the subfiles of the .xsd file.
3. Delete the .Designer.cs or .Designer.vb subfile.  It will probably be the first of three subfiles.
4. Reopen the .xsd file to regenerate the .Designer.cs/.vb file thereby restoring your design view.

If that doesn't work you can always try a Visual Studio repair of some sort, but remember to backup your Visual Studio settings and any Code Snippets you've tweaked.