I finally beat the Guardian Dragon. It told me taht I "may not rest at this time." I'm guessing that that's because my party was really scattered. Then, while I was getting my stuff organized to go back through the portal and find the next battle, I got a message at the bottom of my screen saying that the portal had closed, and that I was trapped in Cluth! I then had the option of either loading an earlier game or quiting! I want to beat the Guardian Dragon for the exp. and the hit point bonuses and resitance bonuses that you get, but I don't know what to do about this.

The Portal is on a timer (that starts after the Guardian is defeated) and if you're not fast enough it closes and the game ends.
However, the game makes a save just after you beat the Guardian so you should be able to start from there. Check the various auto-saves at the beginning of list of saved games - it should be there.

Thanks a bunch! I figured it out and was able to make it through the portal w/ the autosave without having to fight the dragon again! Stangely though, I was able to "rest" without the portal closing. So much for a timer!

For me the rest after the Guardian was automatic and the timer started after that.