I have not been able to figure out why sometimes my admin bar disappears from view when I am logged in. My workaround has been to open up a different browser, or hit refresh many times.
I've had the issue in Firefox and Chrome. I usually pull up IE as the workaround. I just had the issue come up in FF 9 with no extensions, I refreshed the page and it appeared, but sometimes this fails to work. This has been happening for a year or so, so it isn't just the current version.