Default Problem in Galaxy Mega GT-I9200

Hi , i have a problem in i9200

can u help me pls ??

i had a problem in wifi after rooting it ,,

then i installed a rom with 4 files " PDA + BootLoader + Modem + CSC "

and then wifi worked good .. then i tied to root it again and i had the same problem ,, after searching at the internet ,,

the problem is "my boot loader is locked "

but i don't know anything about unlocking it ,, plsss tell me how i can't use my phone plsss"

pls gimme links