SystemImager 2.0.x and earlier didn't maintain the bootable flag in the partition table. This worked fine in most cases, but in some cases this leads to an unbootable system. To confirm that this is the problem, boot your system from rescue media, and set the bootable flag on your boot partition using cfdisk or another partitioning tool. If this allows your system to boot, then you must upgrade SystemImager and regenerate your autoinstallscript(s). If for some reason you can't upgrade, then check the following:
Be sure that you are using the latest version of SystemImager and that you are using the autoinstalldiskette image that comes with that version. Note that the version numbers may not match. See the VERSION file.