I have had my Garmin Forerunner 945 for a while but recently (last 4 months) I have had issues downloading new updates when I plug my watch into my PC and connect to the Garmin Connect Web. I could not update my watch even with simple updates! I also had changed my music playlist and wanted to download the updated music playlist to my watch but was unable and go the same issue.
The issue I ran into was every time I tried to download the update onto my watch, my watch would shut down (all the way to a blank screen) and reboot, and would disconnect and reconnect to Garmin Connect. This would continue until I finally unplugged the watch from my PC or closed the Garmin Connect app on my PC.
I scoured the internet trying to figure out why my watch would keep rebooting every time I tried to download a new update from my PC. It was not until the most recent "map update" that I realized what the issue was! I had too much music downloaded to my watch (over 300 songs) and only had about 2GB of memory left on the watch. What I realized was that the watch just did not have enough memory left to download the new updates! When I erased over half the music from my watch, the issue disappeared and I no longer had issues downloading updates to my watch!
So, if you run into a similar issue of your watch rebooting and disconnecting from Garmin Connect, consider seeing how much music you have downloaded to your watch...you may have to consider deleting some of the music from your watch!!!
I hope this helps, and not keep you frustrated for 4 months trying to figure out the issue!