Due to lack of RAW information on warlock's power, the outcome of such event should be determined by DM. Plausible outcomes are determined by nature of warlock's power (which is determined by dm). If the power comes directly from entity, it would mean power loss, and if it comes from some other place and "pacted entity" simply made that connection, then it is possible that power will remain.
For the purpose of the story DM may decide, for example, that another entity will inherit pacts of original archfiend/archfey, which may make sense depending on circumstances.
To my knowledge, 5e description of warlocks amounts to half of page of text, which gives very little insight on warlock's relationship with pacted entity, and zero information how the pact was made or how warlock's powers function.
Source books from 3.5 (complete arcane and complete mage) speak of warlocks, and there's there's additional mention of them in Fiendish Codex II. However none of this books actually bother to explain warlock powers, which means it is mostly left for DM to decide.
Now, putting aside that killing some fiends was quite hard and you needed to do that on their home plane (otherwise they would reform later - whether it is true or not in 5e remains to be seen), there's pretty much two ways for warlock power to function:
- Warlock gains powers directly from its "master". This is similar to relationship between cleric and deity.
- Their "master" enables them to gain power from somewhere else. Performs a ritual and creates a conduit with greater power source. Like "essence of nine hells", "chaos of abyss" and such.
The obvious consequences are that in scenario #1 if "master" is slain, warlock will lose all powers or most of them. In scenario #2 if "master" is killed, warlock retains all powers, unless connection with actual power source has been severed. If pacted entity is archfiend, it is reasonable to expect that there's some safety mechanism that terminates pacts once archfiend is dead.
The main difference between 3.5 and 5e warlock (aside from warlocks having spellslots now) is that in 5e warlock is in service and "master" checks on the warlock periodically. That is much closer relationship than what 3.5 warlocks had and is somewhat similar to cleric. Which means that complete power loss upon death of "pacted entity" is more plausible in 5e than it was in 3.5e (where infinitely recastable spells suggested connection to some infinite power pool).
From the perspective of the story, another fun thing a DM could do is to make some other entity inherit all the pacts of slain entity. It makes sense if archfiend you pacted with was slain by another archfiend. It makes less sense if it was slain by someone else.
I saw this kind of plot in action, it was a big shock for warlock character to discover that the entity he planned to kill was already slain by someone else and that NEW fiend holds warlock's pact.