Check out your device - if the 1st part is there, then either the transfer was interrupted, or the FIT files is dammaged. Try syncing over USB and Garmin Express. If it does not help, load the FIT file from the device manually (from //GARMIN/Activity) and try importing it to GC Web (https://connect.garmin.com/modern/import-data). If it fails, repair the file with the Section Remover of FIT File Tools (without removing anything).
If the ativity is not on the device, then you have forgotten to save it, or there is no room left on the device.
