You'll need to get the nameplate ring off to expose the screws underneath. You'll need some sort of friction tool. I've found a roll of electrical tape is
the perfect diameter for many lenses and provides ample grip.
I've not disassembled that lens, but my guess is that a screw has come loose inside. Remove the nameplate ring and proceed slowly. Document
the disassembly to insure you can get it back together, (and to contribute to the
lens repair database!).
Indeed, you might want to consult the
database to get an idea of what you might encounter.