Delete the files from your iCloud account. This is not the storage on your iPad, but the Internet-based storage tied into your Apple ID (& accessible from other devices). Like any cloud-based storage, you have a set limit on how much you can store...& you've used up your storage.
Go through the files in the iCloud storage, delete what you don't need (i.e. if you have 5 separate sets of backups, you really only need the last 1 or 2 version), download to your PC the files that you want to keep but don't need immediate access to via the iCloud account, & you'll be set.