Yeah, that sometimes happens for some reason, and I can't for the life of me ever figure out what the cause actually is.
First, check on your GMail account online (using your Mac) and see if there really is an email somewhere. This is the easiest fix. If you find it, delete it or move it somewhere, or read it. Hit Menu then Refresh in the GMail app on your phone.
Second, if that didn't work, in the GMail app on your phone, press Menu, Settings, Labels, and set Number of days to sync to 999, and then in the same menu press your Inbox and set Sync all. Sometimes you have old emails that the GMail app doesn't bother downloading because it's so old, and this will force it to download everything (or at least every email with a date less than 999 days old).
If you still have an email in your Inbox that you can't see, you can reset the data GMail has downloaded by going to Home -> Menu -> Settings -> Applications -> Manage applications -> Google Mail Storage -> Clear data, then go back into the GMail app and wait for it to re-synchronise. That will get a fresh copy of what's on the GMail server to your phone.