When you turn on iCloud Photos on your iPad, any photos on the iPad that were placed onto the iPad by syncing them from a computer (Mac Finder or iTunes in Windows or Mac) are deleted from the iPad. The photos are deleted from the iPad but not from the computer they were synced from.
Normally, those photos would still be on your computer (assuming that you didn't delete them from there) and you could manually upload them from the computer to the Photos app at iCloud.com and they would then sync back to the iPad via iCloud Photos. You could also AirDrop them from the computer to the iPad or turn on iCloud Photos on the computer and that would result in them being synced back to the iPad.
This is also discussed in the following excerpt from Get help with iCloud Photos - Apple Support
Any photos or videos that you copy to your device through iTunes or the Finder are not copied in iCloud Photos.Â
Upload originals from your iPhone, iPad, or iPod touch to your Mac
To upload original, full resolution versions of your photo library to your Mac, turn on iCloud Photos on your Mac.Â
Copy photos from your Mac to your iPhone, iPad, or iPod touch
- Make sure that you're signed in with the same Apple ID on your computer and your device.
- Unlock your iOS device and turn on AirDrop.
- Open a new Finder window on your Mac and choose AirDrop in the sidebar.
- Drag the photos to the icon for the iOS or iPadOS device in the Finder window.
Upload photos from your PC
Learn how to upload photos and videos to iCloud Photos