mjhjmtu

Dissonant whispers (PHB, pg. 235) says:

The target must make a Wisdom saving throw. On a failed save, it takes 3d6 psychic damage and must immediately use its reaction to move as far as its speed allows away from you. The creature doesn't move into obviously dangerous ground.

If someone holds their action to cast this with the trigger being to cast it on a hidden enemy when they reveal themselves, then when the enemy does reveal themselves, and the readied spell is cast, and the enemy fails their save, they must then immediately use their reaction to run away as far as their speed.

However, since this is now happening on that enemy's turn, do they still have their movement for that turn, or did they just waste it running away as per the spell's effects?

In other words, enemy normally has 30 ft movement. If the spell forces them to run 30 ft. away during their turn, can they run back or do they now have no more movement left that turn?

The enemy will have used their movement.

The reason is fairly simple, given a clear look at how movement works: speed is defined as the amount of movement you may use during a turn (unless otherwise noted). Since they’ve used movement equal to their speed, they’ve reached the limit and can no longer move.

The way to think about movement and speed is that you count up as feet of movement are taken until speed feet moved.

No, the affected target can still move because the spell used its reaction, not its movement to move

On PHB pg. 189, under Your Turn, emphasis mine:

On your turn, you can move a distance up to your speed
and take one action. You decide whether to move first
or take your action first. Your speed—sometimes called
your walking speed—is noted on your character sheet.

On PHB pg. 190, under Reactions, emphasis mine:

Certain special abilities, spells, and situations allow
you to take a special action called a reaction. A reaction
is an instant response to a trigger of some kind, which
can occur on your turn or on someone else’s... If the reaction interrupts
another creature’s turn, that creature can continue its
turn right after the reaction.

So in your scenario the affected target would have used its reaction to move, but then it would still have its action, potential bonus action and movement to be used during its turn. If the spell had stated that the creatures movement was reduced to zero after it moved, then it wouldn't be able to move. Since it doesn't, there is nothing preventing the creature from using its movement to return to where it was.

If it helps: There are 4 actions that a character gets per round. Action, Bonus Action, Movement, and Reaction. Whether you qualify for a bonus action or a reaction is dependent on circumstance, and if something uses one of those, they're independent of the other 3.

So for example: A Battlemaster Fighter using the Ready action to allow a Rogue to slip past an enemy on the Rogue's turn could state: I'm going to Ready my Maneuvering Strike and shout at the Rogue, "Run! I have him distracted!" The trigger will be as soon as the Rogue acknowledges the plan.

So now on the Rogue's turn, the Rogue says, "NOW!" The fighter triggers Ready Action, uses Maneuvering Strike and selects the Rogue as the recipient. The Rogue gets to move up to half their speed using their reaction, so the Rogue moves 15 feet, not triggering an opportunity attack from the enemy. Then the Rogue uses his movement to travel a further 30 feet past the enemy, Cunning Action Dash for another 30 feet, and Action Dash for yet another 30 feet, totalling 105 feet of movement away from the distracted foe.