I've been using the Jawbone UP Band for a bit now and ran into an issue where I could not get the band to sync with my iPhone. It also appeared that the function button wasn't accepting press input either. After searching for an answer I came across the following solution from Jawbone.

Soft Reset
If your UP band’s button, lights, or vibrations are not working properly, or your band won’t charge, you can perform a Soft Reset to reboot the processor in the band, without erasing your user data.

Follow these steps:

Plug your USB cable into a working/powered USB port.
Press and hold the button on your band.
Continue pressing the button while you plug the band into the USB cable.
When the band’s lights flash yellow and red, release the button.
Sync your band with the app.

Hard Reset
To erase all user data currently stored on your band and return the band to its original factory settings, perform a Hard Reset. A Hard Reset may also help fix any band performance issues that a soft reset is not able to fix.

Follow these steps:

Make sure your band is charged.
Press the button on your band 10 times.
On the 10th press, hold the button for 10 seconds until the red sun lights up.
Sync your band as you did upon first use.