Most likely, you just need to do a hard reset on the Slingbox. This will clear everything back to factory defaults- including the hardware admin password- so you can start over fresh.
Here's a support doc that covers the procedure:
Once your Slingbox is reset, run the Setup Assistant again. It should prompt you to choose new passwords. Go to http://setup.slingbox.com from a Windows computer on your home network to get to the Setup Assistant.
(If you're using the Mac or Windows full desktop version of SlingPlayer, you can do it in there as well)
Hope this helps,
...when I first set it up, I had some problems when trying to access the box while away over the Web.
That's a problem as far as Setup Assistant goes. You need to be on the same network as the Slingbox for that to work. After that though, it's possible watch and reconfigure things over the internet just fine.
Unfortunately, if your Slingbox admin password isn't working a reset is pretty much required. The Setup Assistant normally handles configuration for remote viewing as part of its process, so maybe it'll go smoothly this time.
I hope you don't have any headaches once you get back in, but if you do you can always post about it. Someone ought to be able to help.