I am guessing that a setting in your video source was accidentally changed or has become corrupted, e.g. as a result of a firmware update.
If you don't see a problem with its settings, please post:
1. What is the aspect ratio of the content you are viewing (4:3, 16:9, other)?
2. Make and model of set-top box or other video source?
3. Can a person at the Slingbox location verify proper video output from the source?
4. Output resolution from video source, e.g. 480p?
5. Video source output aspect ratio "TV aspect ratio"?
6. Any settings in video source such as letterbox, pillarbox, zoom?
7. Fail with both desktop and web-based player?
8. Post a screenshot, taken from the standalone player (hold Alt and then hit Print Screen), using the initial window that opens (don't resize window or go to full screen). The screenshot should show a picture such that the clipping is obvious, e.g. text is missing at left or right. It should also show whether the aspect ratio is correct, e.g. have one or more people in the picture. If needed, post two shots. Don't process the shot(s) in any way that affects picture size.
9. In the destop player, hold Alt and Shift then press I (for Info). A Statistics window will open. Post the values of Video Width, Video Height, and Output Aspect Ratio.