You need file recovery software. I've got no idea what's out there for Android, but you can always put the flash drive in an adapter on a PC or Mac, and there's gobs of known-reliable software on those platforms, usually with 30-day personal free trials.
You might want to do a drive copy, and make a duplicate of the flash drive, just in case the recovery goes bad. That's how professional do it.
I'd also avoid going to any big-name store chains for this, as I've seen them wipe out drives they were supposed to recover.