I've had a run of success lately with this issue. I power down the watch (not to the point that it does a hard reset, though) and then attach the charging lead to the watch, before plugging the other end of the lead into my iPhone charger. This wakes the watch immediately, it vibrates and displays the message "Charging". Once at 100%, I simply unplug the lead from the charger and the watch is ready to go. No loss of time or data. Has worked for me three or four times now. Seems that plugging the watch into a USB socket on the computer was what it didn't like.
Worked perfectly thanks!
You don't need to switch it off if you want to charge with your phone's charger. Only if you want to use the PC. Meanwhile I avoid charging with the PC although I have to use Garmin Express every few months for syncing my activities. With Garmin Connect Mobile they all stay on the watch until I get the message with low memory. After syncing with GE everything is fine again.