Can you boot back to the previous BE? Also what system is this? Where does it hang?
Have you tried a boot -Vv to get the verbose output?
Is this zfs or ufs?
Have any SAN devices changed, or do you have badly configured network interfaces? Typically these things cause boot hangs. It's almost always something in the device tree.
Are you sure that the server hangs and doesn't need more time to boot ?
What Hardware are you running it on? and does it hang after it starts booting from disk or before?