Returning to work after 4 years - what to do about not getting selected due to a lack of knowledge?

This is about my career; I left my job in 2014 due to some family problems. But now I want to get back to work.

I have attended many interviews but have not been selected due to a lack of knowledge. Please suggest what I should do.

"But not getting selection due to luck off knowledge."
I presume you meant LACK of knowledge. The answer is clear from your question. Please join a certificate course or another suitable course in your primary skill. Once you gain some knowledge about your basic area of work, then probably you will get jobs.