iMac chimes but nothing on screen

PLEASE HELP!!!

My 2013 21.5 inch iMac's screen is just black when I turn it on. I performed an ssd upgrade and disassembled the motherboard to put a new pram battery and new thermal paste on the chip. the display was already cracked before but now the display does not even show anything it just stays black.

when I turn on the Mac there is 3 led lights turning on but the 4th one does not turn on. when I connect the iMac to a dell monitor with Mini DisplayPort to hdmi it shows an image and everything works as normal but also from boot the fans go to maximum speed even though it didn't used to and I cleaned it while upgrading. When I load into macOS, the sound icon is greyed out even though it used to work fine and the chime at boot is loud and clear. I have no info on the iMac that I need so please tell me if I need to wipe the iMac if necessary. I would really appreciate any help because I was planning to sell the iMac for another Mac.

Please help me because I've tried everything and I'm out of ideas.