Due to the lack of RAW information on a warlock's power, the outcome of such an event should be determined by the DM. Plausible outcomes are determined by the nature of a warlock's power (which is determined by the DM). If the power comes directly from a given entity, it would mean power loss, and if it comes from some other place and the "pacted entity" simply made that connection, then it is possible that the power will remain.
For the purpose of the story the DM may decide, for example, that another entity will inherit the pacts of the original archfiend/archfey, which may make sense depending on circumstances.
To my knowledge, 5e's description of warlocks amounts to half of a page of text, which gives very little insight on the warlock's relationship with their pacted entity, and zero information how the pact was made or how the warlock's powers function.
Source books from 3.5 (Complete Arcane and Complete Mage) speak of warlocks, and there's additional mention of them in Fiendish Codex II. However, none of this books actually bother to explain warlock powers, which means that it is mostly left for the DM to decide.
Now, putting aside that killing some fiend 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 a warlock's 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. The warlock performs a ritual and creates a conduit with greater power source, such as "essence of the nine hells", "chaos of the abyss" and such.
The obvious consequences are that in scenario #1 if the "master" is slain then the warlock will lose all powers or most of them. In scenario #2 if the "master" is killed, then the warlock retains all powers, unless connection with the actual power source has been severed. If a pacted entity is the Archfiend, it is reasonable to expect that there's some safety mechanism that terminates pacts once the archfiend is dead.
The main difference between the 3.5 and 5e warlock (aside from warlocks having spellslots now) is that in 5e the warlock is in service and the "master" checks on the warlock periodically. That is a much closer relationship than what 3.5 warlocks had and is somewhat similar to a cleric's relationship with their deity. This means that complete power loss upon the death of a "pacted entity" is more plausible in 5e than it was in 3.5e (where infinitely recastable spells suggested a 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 of the pacts of a slain entity. It makes sense if the 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 the warlock character to discover that the entity he planned to kill was already slain by someone else and that a NEW fiend held the warlock's pact.