If the spell ends, all of its effects also end; the attacker could make more attacks on their turn
The slow spell states:
[...] Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.
An affected target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn [...]
And from this text we can see that if the spell ends, then its targets are no longer affected by the spell. This means the restrictions put on them by the spell no longer exist. For this question, that means the attacker can now make more attacks during their turn.
Another way to think about this is to note that the spell didn't come into effect when you took the Attack action, it was always there, limiting your options. Removing the spell removed those limits. The additional attacks themselves were never removed, you simply weren't allowed to take them. That changed when the spell ended and they once again became available for use.
We know that the spell ends when the caster loses concentration not only intuitively but also from the rule on "Concentration":
Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.
- Player's Handbook: Chapter 10 "Spellcasting": Concentration (page 203)
I honestly can't see another way to think about this because it would lead to the seemingly illogical conclusion that the spell is no longer affecting you (its duration has ended) and yet it still has an effect on you (you can't make more attacks).