More likely than losing the history, is that you are logged into a different account than originally. Try signing out, and signing back with the original credentials. If it does not help, try following the instructions in the document Data is Missing from My Garmin Account After Syncing
Thanks trux, you were right, they'd decided to go from googlemail to gmail on my email address, which i've never had an issue with before, the only thing now is it's not uploading my recent activities from the watch even though i've turned every thing off and on, removed the device, reinstalled everything etc but no joy
