You can access your files by Files app > on my ipad > Artstudio pro!
Also not LuckyClan but I am a developer, and I'm suspecting something hiccuped when the file was deleted. .artstudio files have a thumbnail separate from the layer contents and between crashes, running out of battery, or random miscellaneous weather events sometimes things do hiccup and it's likely your deleted file's layer contents replaced the file that you were working on. Probably no way to get that file back unless you have a backup.
It's a shame artstudio doesn't move deleted files to recently deleted section in the Files app, and rather outright deletes them with no chance of retrieval other than external backup.