There is an option in the menu to reset SYNC to default, but if that doesn't work I'd recommend pulling the fuse under the dash (I think SYNC is fuse #14) and resetting it that way. You'll have to pair up whatever bluetooth devices you use again, but everything will be reset and working normally.
I had the case about a month after I bought the car that SYNC screwed up and wouldn't connect to my phone. Reset it this way and haven't had a problem since.
Definitely reset via the menu FIRST, pull the fuse as a last resort.
2011 SES 5spd #006 >> SR #213
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