My devices have been hacked. What do I do?

i was using my ipad a short while ago when suddenly it locked itself, and was askiwhich I'd never previously set up. I went to check my phone and there was a message on the screen (it's still there) saying that my device(s) had been hacked by 'Oleg Pliss' and he/she/they demanded $100 USD/EUR (sent by paypal to ****) to return them to me.

I have no idea how this has happened. I am not aware of having been exposed to malware or anything else, although i did recently purchase some new apps - perhaps one of these has something to do with it? I don't know. I am not sure what avenue has been used to reach my devices - I'm about to use my husband's laptop to check through some of my accounts (gmail, etc) and see if there is any clue there.

Has this happened to anyone else? What can or should I do? Many thanks