I'm looking for a single word to replace a 'lack of response'
Solution 1:
"Unresponsiveness" works if you really need it to be a single word.
Solution 2:
Perhaps
The silence was deafening.
Solution 3:
If OP doesn't like indifference, how about
apathy - lack of interest or concern, especially regarding matters of general importance or appeal
Solution 4:
How about "non-response"? Would that work for what you're doing?