Hard Drive Failure after Battery replacement

Hello Fellow Techncians

I was wondering if you could help me

I have replaced a battery in a ipod 4th gen with Click wheel, as with all repairs, i very carefully opened it up and removed the hard drive and placed it in a safe place, there was no bumps knocks etc

After replacing the battery and powering it up i get the dreaded folder icon with the exclamation mark

i have tried reseating the hard drive multiple times and i have also tried putting it into disk mode. it gets into disk mode but when i plug it into a PC (i am not using a fire wire cable just a standard ipad 2 cable) it resets and i get the sad face icon

im perplexed as the hard drive was fine before now there is no activity

also when the apple logo comes on there is no back light

Please can you help