Unfortunately, this has come from a high-rep account and that means a few people may be taken in by it. I hope something can be done about it before anyone really is taken for a ride.
If you get this kind of message, I strongly advise you go to steemit chat and ask there, rather than replying to a message in your wallet!
Steemit Abuse Chat - <--- this link is safe to click (but no need to take my word for it, research it).
The Message I got in my wallet:
ACCOUNT BLOCKED: We have detected unauthorized activity in your account. Your account has been BLOCKED for your protection. Please Contact Account Security: Redacted Link for safety
https://steemit.com/@samstonehill
Unfortunately, it looks like 
This is a better post too, more information.
https://steemit.com/life/@exyle/lvl-70-account-samstonehill-sending-spam-warning-account-propably-hacked