Please help I ran a half marathon this morning and my Garmin Forerunner 35 has only uploaded half of the run. It initially wouldn’t sync nor come up on Garmin Connect! We have managed to import the file but only half has come through. Please help!

If importing to Garmin connect didn't say the file was corrupted, then maybe it only contains half if the run. There are tools that can open fit files and you can check the time at the beginning and the end

Can you zip the file and post it here (Insert > Image/video/file)? Or use a file-sharing service (Google Drive, Dropbox, https://www.file.io, etc.)

Then we can try to repair it.