recovering a "lost" database

i have a database that was "lost" -- the database is still there (i can
see the files on disk in the database's directory), but when i try to
connect to the database, it says that it doesn't exist in pg_database.
what happened is that the database got dropped while the drive with the
data on it was unmounted, but now that i thought about it, it would be
nice to have a backup of it if i can get it.  is there a way that i can
manually add it to the pg_database table (and are there other tables i
have to mess with?) so i can access it again?

thanks,

jeff