Not all monitors have a degauss button on the control panel. Many CRTs are degaussed when you turn them on. You can try turning it on and off a few times to see if it helps. Also, for the monitors built-in degauss to be most effective you should let it sit turned on for 5-10 minutes while components recharge.
To be cost effective, your money would be better spent on purchasing a new monitor vs buying a commercial degausser. I would hope that this kind of thing does not happen that often.

Good luck!
~Zip