I too had this problem with my watch this morning, it was stuck saying "installing system software update"
I plugged it into my computer, opened Garmin Express and after syncing it said my watch was up to date with firmware 4.10. So then I unplugged the watch and it came up with an option to install 5.10 now or tonight, selected now and it just installed fine.
Was worried my watch had bricked itself, but all is well now!