I had a loose A28/2.8 too. There are three lens elements in an aluminum carrier, forming a group in front of the aperture. The carrier is threaded, and screwed into the lens body. So screwing it back in should work. The loose part would normally move in and out when the lens is focused, so as long as it can't shift up and down any more, the lens should perform fine.
There is a beveled ring inside of the filter threads, threaded into the filter threads. If you can unscrew that, you should have a little more access to the group carrier to tighten it more.
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