cm 12 stuck at boot loop! topic

i was flashing build 18 on s4 when my phone failed to flash.so when i rebooted it shows splash screen of my phone name but in boot animation only blue notification light with black screen,i tried to reflash earlier builds but no change pls help and tell what to do? I had s4 working on 10 1 2015 build of cm12 earlier .. :(