Problem Booting (Bad Super Block)

[Rishi Chopra]

I've started having problems on my FreeBSD box; I've installed FreeBSD 
on this system at least 5 times before without any problems.

This time, the installation took many attempts, and the following error 
message occurs when I attempt to boot up:

/dev/da0s1a:  BAD SUPER BLOCK:  VALUES IN SUPER BLOCK DISAGREE WITH 
THOSE IN FIRST ALTERNATE
/dev/da0s1a:  UNEXPECTED INCONSISTENCY:  RUN fsck MANUALLY

Automatic file system check then fail, the boot process aborts, and the 
system starts in single user mode.

How can I fix this error so that the machine boots normally?  I am 
unable to run fsck from the prompt by just typing 'fsck'....