That symptom, may indicate several things.
Did the USB stick get bumped - that can affect the seating and communication it has with your head unit? IF it is not seated - it can't tell the system something is there (I'm Here!)
If it's an iPhone or Android player, make sure you have the player on - the USB port can recharge, but the engine needs to be started - just can't put the key in or hit Start/Stop just to tun on ACC
Although nothing too serious as far as drivability the issue sounds more like what I call "French fry syndrome" - it's where dirt, debris or finger grease gets into the keypad and shorts out the key it's lodged by or underneath - forcing the entertainment system to "think" someone's holding a key down.
But if there is a chance that you have a stuck "ok" button key or some key on the pad both left or right side of the main volume control - just do a good cleaning with a mild soap (dawn or dishwashing type - not a lotionized or aloe based) weak solution on a simple dishwashing rag and wipe down the keypads thoroughly to try and free up the keys so you can check operation. Don't "soak" the keypad, just wipe it - and vacuum too if you have a unit that has a crevice tool or soft brush to try and pull out or reseat the keys on that pad properly. May have to repeat the process - even (no offense) body hairs and fingernails as well as the body oils, can affect the operation of the keypad - some body lotions are pretty aggressive when it comes to softening plastic.
Once that's done, also look for deformed trim around it - it may indicate a keyboard failure from physical damage onto the sunlight warping the dash keypads rendering them unusable.
If the USB port is still ok, and no physical damage to the socket - try a different USB "stick" to see if the SYNC system will try to recognize it. Although unlikely - a pulled fuse or interrupted "index" function by pulling the battery trying to attempt a reset - can damage the USB stick and if SYNC sees the "ID" and file system of the stick - it may think it's corrupted and not even try to use it. Skipping over the function and returns back to the previous one it defaulted to.
Just some ideas from a Sync user...
