If upon uploading an activity from your Garmin device you see an error message telling you that your upload was a "partially corrupted file, could be truncated," this means that there was some recording error on your device, and the file is at least partially corrupted
Strava has retained as much information from the file as possible - and that data will now be present on your Strava account. If a portion of your activity is missing from the upload, there, unfortunately, isn't any way to recover the remainder of the activity.
In either case, it might help to perform a reset on your GPS device - although these errors do happen relatively infrequently. If you have any additional questions please proceed with creating a support ticket.