If the file was inadvertently deleted, you have a slim chance of recovering the .fit file using a file recovery tool. However, the more you use your watch the less likely it is recoverable, as those are overwritten. If recovered, it is likely that the file is corrupted, possibly beyond repair, but you could zip it and post it here if you want us to try to repair it..
Turn the watch off, attach it to your computer, then only turn it on when you're ready to try one of the file recovery tools
Activity files located in the \GARMIN\ACTIVITY folder of the device.
There are probably more but here's a list of recovery tools:
If you're able to recover it, then try a manual upload to Garmin Connect - How Do I Manually Upload an Activity I Recorded to Garmin Connect?
If you get any message other than Import Complete, then there is a problem with the file being corrupt. Attach it to this thread. We can take a look to see if it is repairable.
I found some files but have been unable to fix them through FitFileTools or Garmin.stevegordon.co.uk
Are you able to help at all?
Oh sorry,I updated the link: