One of my sites also experienced something similar about a month and a half ago. Two of my inner pages just disappeared from the SERPs and they both were ranking on the 1st page. I couldn't figure out why because the content was unique and of high quality (in my opinion).
I didn't do anything drastic, I just left it as is - I didn't even backlink those pages! A week ago after been in the abyss for over a month, both pages returned to the 1st page and they are now ranked even higher than they were before.
Keep in mind that these pages have PR, so they were not new or anything. I still haven't figured out why this occured, so, I guess it will remain a mystery.
Just wait it out and see what happens! Concentrate on another project to take your mind of it - that's what I did!