From time to time, depending on your hardware, grml might have startup issues.
If it hangs on udev discovery, it tells you about the noudev boot flag. If you use that and it hangs later on, try mgag200.modeset=0.
Sometimes nomodeset (disabling the framebuffer) helps, too.
If it boots almost completely through, you might try pressing d or u, depending on the language you want to use, followed by x so it boots the xserver. This sometimes works even if the text console does not.