Do you have a pre-existing iCloud or iTunes backup? If not, you may have lost your locally stored data. If you do have a backup, the easiest solution may be to first restore your iPad to Factory Settings - which will install a fresh copy of the most recent version of iOS/iPadOS that is supported by your iPad - and then restore your data from your existing backup.
Restore your iPhone, iPad or iPod to factory settings – Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
You’ll need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS).
Download and use iTunes for Windows – Apple Support
Update to the latest version of iTunes – Apple Support
Use of a PC or Mac computer is unavoidable. If you don’t have access to a suitable computer of your own, that of a helpful friend or family member will suffice. Alternatively, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be delighted to assist.