The BSD disklabel lives in the slice, so the one in sector 1 (counting from 0) is the stale one and the one preventing you from booting. This will solve the problem. (May not need the second and third lines, depending on your setup, ad0 is the hard disk you boot from, make sure you check that is correct.)
# dd if=/dev/zero of=/dev/ad0 count=1 oseek=1
dd: /dev/ad0: Operation not permitted
# sysctl kern.geom.debugflags=16
kern.geom.debugflags: 0 -> 16
# dd if=/dev/zero of=/dev/ad0 count=1 oseek=1
1+0 records in
1+0 records out
512 bytes transferred in 0.000164 secs (3121343 bytes/sec)
Reboot again, all should be OK.