It's not clear exactly what type of deletion you are talking about, but...
If you have deleted photos within the photos app, all you need to do is go to Albums > Recently Deleted and recover the photos back to your library from there. Tap the “select” option in the top right corner, tap the photos you want to recover and tap “Recover All” option in the bottom right corner.
However if you have chosen to delete the library from the cloud, you need to reverse the deletion at Settings > iCloud > Storage > Manage Storage > iCloud Photo Library.