Do you know how to restore backup files for story? If not, there's a video in the spoiler below, but you'll have to make a couple adjustments for it to work with arcade. You'll want to find the ArcadeMode_[some numbers] folder, take the most recent file, copy it to the "Secrets of Grindea" folder, delete the "arcademode.sav" file and rename the file you just copied to that.
If you load the game up and it doesn't look like you've got anything back, try again but with the second most recent file. The amount of files you have to go back may vary depending on how long you have played arcade after your save was deleted. If, after 5 files (and you haven't played after the reset for longer than about an hour) come back here and ask someone else who is more familiar with reloading arcade backups
Hope this helps. It's a bummer having a save disappear, let alone the many, many hours spent in arcade.