There are two types of answers defined for HARQ mechanism:
ACK – meaning – I received the transport block and it’s correct;
NACK – meaning – I received the transport block and it’s not correct.
There’s of course the third option – namely, no answer/feedback. It may mean:
either: Rx has received Tx transport block, and send the feedback, but Tx didn’t get the feedback;
or: Rx has not received Tx transport block at all.