Vivo y71 lcd blank issue

Hello everyone i have vivo Y71 received from customer in blank condition . but when i open back plastic cover after that open screws and metal sheet just reconnect all lcd flex and battery and charging board flex . then phone working ok display also working perfect . but when i try 2 fix back plastic cover display gone blank . without back cover phone display working perfect touch everything ok. main problem was when fixing back plastic cover then display blank otherwise working perfect .