iMac black screen after replacing blade ssd & 2.5 sata ssd.

so basically gave a replacement run with guides i found here & youtube. my iMac is late 2015 retina 5k 27” with default option(i5 3.2ghz & 8gb ddr3 ram). went down all the way in taking out every pieces to fit 1tb of blade ssd slot first, then i assembled it back with given steps and installed 2.5” ssd while doing left speaker part (i have taped 2.5” ssd on back plate of imac since slot was too big for it).

then a screen was loaded on top with its display cable(one with flip lock i assume) and powers, for some reason i was struggling big time with those flip lock display cable since it wasnt fitting in any ease and whenever it felt like it was inserted for a lock to be flipped and secure, it would come off in ease with such a little touch or so. anyways, i was somewhat able to fit it in with help of gaffers tape then could drop the flip lock but still seemed it wasnt that of a tight. i assumed maybe its like that in first place so moved on, but got a black screen :( wouldnt even give a bit of flicking light that it would hint me screens are properly attached. and for some reason fan are blowing up like mad (havent installed os yet so maybe that could be reason).

any thoughts i could try looking back in for a fix??

hope its not a logic board’s display port’s failure or screen itselfs. any help would be great!!!

edit:- have tested with my external lcd monitor to see the boot logo and it did show so guess my problem is about imacs built in display :(