To be honest there are a million things that could be wrong.
What you need to do is try to debug each section of the video path rather than the entire thing all at once.
I suggest first checking that the AM335x output timings are correct. Focus on the h-sync, v-sync and AVID signals (don't worry about video data yet). If these are correct then check the THS8200 output H & V to make sure that they are also correct. If the AM335x is correct but the THS8200 timings are not then it is a THS8200 configuration issue
If the THS8200 output timings look correct then does the monitor correctly detect the video mode (again, do not worry about the actual displayed image, only whether the timings are detected correctly). If the timings are detected correctly, but the displayed image is not correct then start looking at the frame buffer configuration etc...
If the timings look correct then try enabling the AM335x output test pattern generator. If this displays correctly but the frame buffer image does not then it tells you everything from the DSS onwards is OK. If no image is displayed then the DSS and/or THS8200 are not configured correctly.
Basically, break the problem down in to manageable chunks and debug each stage in the path starting at one end and moving up the path.
BR,
Steve