Try zooming in and out.
Zooming didn't seem to help at all. The image is just stuck a little titled and the checkered background is showing around the edges.
From your description, it sounds as though you:
1. Have View > Show > Grid checked.
2. Chose the Ruler tool to draw reference line along a tilted object and then clicked Straighten Layer in the Options bar.
3. Then, you tilted the image, and now you want to return to the pre-tilted version.
If that is the case, just repeat Step 2.
If, for some reason, you want the grid to be positioned along a special line in the image, do this:
4. Choose Image > Canvas Size ... click Relative and add 1" to each dimension. Then:
5. Use the Move tool to align the image with the grid.
6. When you are satisfied with the result, Crop the image to its former canvas size.