you changed configuration "uboot"?, resolution, frequence????
You need set variable vid_dev0, example:
setenv vid_dev0 hdmi,1280x1080M@75,if=RGB24
setenv vid_dev0 hdmi,1280x960M@75,if=RGB24
setenv vid_dev0 hdmi,1024x768M@75,if=RGB24
I have sucess from vid_dev0 hdmi,1280x960M@75,if=RGB24