I recorded my screen using ffmpeg and converted from mkv to mp4 and uploaded to youtube. But all I see on the video is gray nothing else. The audio still plays though, I tried converting to numerous formats but no luck.

Here the code for recording the desktop (If it helps)
ffmpeg -f x11grab -r 25 -s 1366x768 -i :0.0 -f alsa -ac 2 -i pulse -vcodec libx264 -preset ultrafast -crf 0 video.mkv
How I tried converting:
ffmpeg -i foo.mkv bar.mp4
I also tried using parameter -vcodec but no luck.

Thanks In Advance