Apple users in Australia are reporting their devices are locked with a ransom message from Oleg Pliss asking for $50-100. If you don’t have a passcode on your device, set up one now. This blocks the attack. So does turning off Find My Phone.
Oddly, earlier versions of the ransonware said to send the money to PayPal, yet there was no account for “Oleg Pliss” and PayPal would shut it instantly if there were. Later versions of it are using other payment methods.
If you have a passcode for your device, then you don’t have a problem — just use the passcode to get into your device again, and change your iCloud password. Find My iPhone can only set its own code if you have not created your own passcode for the device.