iPod 4th gen screen does not work after replacement
The technical term you are looking for is "it's busted"

If I read the sequence correctly, you disconnected a major component while the device was operating and now it is unresponsive. Some electronic components can be disconnected without damage, others don't like it and express their displeasure by declining to work again.

Bare minimum, you will need another, known-working screen to see if the screen or the main board is damaged. The cracked screen is fine, IF it was actually working before replacement (cracked glass is fine, cracked glass + no display is not fine). Repair centers keep various components handy for exactly this reason.

if the known-good screen doesn't work, then you have fried the logic board. At $250 for a new iPod we have to seriously consider how much more effort we want to put into more repairs.