Well, are you sure someone did not change the wallpaper on your lock screen? Can be viewed in Settings > Wallpapers.
If not, then 'Reset' the device (directions below if unfamiliar w/ the procedure) - good luck & welcome to the forum! Dave
Reset - press & hold the 'Power' and 'Home' buttons simultaneously until the Apple logo appears - this may take a longer time, so have patience; the iPad will 're-boot' - NOTE that none of these maneuvers will alter your apps, files, or settings.