My Garmin Forerunner has given a lot of headaches lately. It has not uploaded my running data to Garmin Connect, causing me to lose data from three runs, including a personal best.
As with most devices, a master reset did the trick. So, if you have this problem I suggest you try the following. It worked for me. Just remember that you will lose all data and there are no guarantees that it will work for you, just because it worked for me. I stole this from the support forum by the way.
To perform the master reset:
1.Power Forerunner off
2.Press and hold Enter and Mode buttons
3.Press and release Power button
4.Press and release Lap/Reset (watch will power off)
5.Wait 3 seconds
6.Release Enter and Mode
The reset on the Forerunner 310XT is only successful if the device finishes powering up, and goes to the setup wizard. Repeat the steps again if the setup wizard does not appear.
Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data once the reset and setup wizard are complete.