I don't think anyone can give any advice until the reasons for easyjet cancelling your booking have been determined.

It is very unusual that any airline would cancel your tickets and issue a refund without informing you or (apparently) even making an attempt to contact you. Even more so given that you had already checked in.

Was there anything unusual or strange about the price, the manner of booking, the card used to purchase these tickets? Anything at all that could have attracted any form of unusual or unwarranted attention from the airline?

If easyjet are unable to substantiate the reasons for the cancellation, then I would expect that they should make some form of gesture to you. I am not sure whether this falls in the realms of EC261/2004, but if you would be happy with compensation/recovery along those lines, then perhaps that is a starting point for negotiation.