have a script to do this:

CONNECT TO mydb;
QUIESCE DATABASE IMMEDIATE FORCE CONNECTIONS;
CONNECT RESET;
backup database mydb to e:\sqlbackups\mydb compress;
CONNECT TO mydb;
UNQUIESCE DATABASE;

During the backup, the server ran out of disk space. We freed up
space. Because it's windows and runs as a task, I couldn't see which process it was...so we rebooted the box...

When backup ran the next night here's the output:

QUIESCE DATABASE IMMEDIATE FORCE CONNECTIONS
SQL1371N Instance or database "mydb" is already quiesced. Quiesce
type:
"Database".

CONNECT RESET
DB20000I The SQL command completed successfully.

backup database mydb to e:\sqlbackups\mydb compress
SQL1035N The database is currently in use. SQLSTATE=57019

CONNECT TO mydb

Database Connection Information

Database server = DB2/NT 8.2.5
SQL authorization ID =
Local database alias = mydb

UNQUIESCE DATABASE
DB20000I The UNQUIESCE DATABASE command completed successfully.

And yet I was able to quiesce and unquiesce ok this morning. If I run the backup again, will the quiesce work? (This db is 120 gigs, so it takes about 5 hours to run and runs at night).

most probably it should work...
the DB was left in quiesce state which was later unquiesced.....
so now it seems its back to normal

ok thanks, if I still have problems, do you think a db2 stop force would be helpful?

no: db2stop will not correct the problem
use : connect - UNQUIESCE DATABASE