trouble with my 28-70 2.8L -only goes to 50mm

I recently noticed that my go-to 28-70L lens will now only zoom to 50mm, and I can rock/tip the front extension about a quarter of an inch...  almost like something has come loose/off inside. No impacts that I am aware of.  Seems to be working fine otherwise though...  Time for Canon service?...  or can this be done as a home repair for the not-to-faint-of-heart?

Thanks for any insights!

Dave

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pro photo-illustration
http://www.DaveSeeley.com/
current kit: 5DmkIII, 5DmkII, 17-35/2.8L, 24-105is/4.0L, 28-70/2.8L, 70-200is/2.8L, 100 macro/2.8L, 50/1.2L, 2x II extender, 8x550ex Speedlites, Phottix Odin Flash Triggers