If that happens, Hard Fork 20 will simply not push through and the mechanisms and rules of HF 19 (the one we are on right now) will be followed. Nothing changes.

It is worthy to note though that in order for HF20 to push through, a supermajority of the top 20 witnesses needs to reach a consensus. 17 out of the top 20 witnesses needs to update their system into the latest one (HF20) before Sept 25 in order for it to be implemented on the whole blockchain.