How can I get data back from a deleted custom field?

Hi Poonum, welcome to the Community. Unfortunately, when things are deleted they are gone. there is not way to recover it short of restoring from the most recent backup prior to the deletion. However, generally speaking the cure is often worse than the disease when considering restores since rarely do you have a recent enough backup. So restoring ends up loosing a bunch of recent stuff. If you do have a recent backup then you can run some queries of everything that was changed since the backup and try to restore and then recreate all of the newly lost stuff. Messy, yes.