What can I do if Solaris won't boot?

At the "Type of Installation: Interactive or Jumpstart" menu,
type "b -s"

Or, after the video configuration, network, time and date you'll notice
one of the menu's has a button: [Exit] Select Exit and, when it asks
you again "do you want to exit?," just say yes.

Once you're at the UNIX root prompt #, you can mount the boot drive with
"mount /dev/dsk/c0t0d0s0 /mnt""
and view anything wrong with the boot drive (omit the "t0" for ATAPI).

[Modified from Bob Palowoda's Solaris 2.4 x86 FAQ]