iPhone sim card locked

WHEN I WAS IN USA I HAD AN IPHONE 4 (version 5.1 ) with AT&T mobile SIM CARD .

NOW I LIVE IN GREECE . I TRIED TO USE A NEW SIM CARD OF A LOCAL (GREEK ) MOBILE COMPANY.

WHEN I INSTALLED THE NEW SIM CARD I GOT THE MESSAGE<<the sim card IS locked>>.

NOW, I can't use the IPHONE 4 as mobile phone !!!

DO YOU KNOW ANY SOLUTION

G.P.NIK