Gmail Messages Disappear iPhone

Hello, I use Gmail for several email accounts. Ive been getting the messages on my 3GS fine, but this morning, all but three of them disappeared from my Inbox (they are still on my Gmail account; I can see them and manipulate them with no problem). In the iPhone settings, I had (inadvertently) set the Gmail accounts Days to Sync at three; I have since reset this to 1 Month. Is that the problem, and will I ever be able to see the old messages on my iPhone again? This seems to have affected the folders and Gmail mailboxes as well. I have my Gmail account enabled for POP and IMAP. This has never happened before to my recollection. Thanks!
iPhone 3GS iOS 4 Gmail