If it used to work and now it doesn’t, this could be a few things in my experience. It might help to know what happened between the last time it worked and now.
First thing I would try is to restart the iPad. If this is still unsuccessful a few options below.
# Verify the adapter you’re using works, assuming you’re using one. Check for any damage, (discolored or bent pins, kinks in the cable if there is one) is free from any damage. See if it works with other Apple products if you have any.
# Has it been connected to other devices? Will it connect to other devices? If yes, I would move any data you don’t ant to lose off the flash drive, reformat it using exFAT if you will be using it with multiple platforms or APFS if you’re only using it with Apple products.