The PHB is a bit unclear when it comes to readied spells in combat. If I lose my concentration do I lose the spell slot? What about if the trigger never occurs, do I get the spell slot back?
You have cast the spell; you lose the slot.
The way readying a spell works is that you cast the spell, and then on a trigger you let it go. Picture an old school Dragon Ball Z battle with the characters charging their powers and then letting them go.
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. (Players Basic p. 72)
Casting a spell "as normal" indicates that you consume the slot. No matter what happens after that, the spell has been cast and the slot has been consumed.
Concentration comes AFTER a spell is cast so you already lost your spell slot by that point. Concentration controls the duration of the spell.
As for the Ready action the rule reads.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
It allows you to take an action or movement as a reaction in response to a trigger. If the trigger never occurs the action never occurs. However spells are an exception.
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
So at the moment you declare the ready action with a spell you have expended the spell slot. Whether the spell is actually takes effect depends whether the trigger occurs and concentration is maintained. If concentration is broken via the conditions in Chapter 10 then the spell does not take effect and ist lost. If the trigger never occurs the spell does take effect and is lost.
The way I envision magic spell slots is that they occupy a certain piece of memory inside the caster's mind. In case of prepared spellcasters, they require time to memorize the spell, and after casting a spell, they forget that particular piece of mind-memory.
When you ready a spell, you cast it as normal. The way the spell behaves from that point on is moot with regards to how it affected your mind. You cast it, therefor your mind's memory for it is spent.
You have already lost the spell slot, so it can't be regained. You can also lose the spell's energy if you fail to concentrate. I see concentration as a caster's way of preserving the energy of a spell. It's already out of their mind and into 'play'. If concentration is broken, or a readied spell is never triggered, it only affects the spell's energy being lost. It doesn't affect your spell-slot; you already lost that when you cast the spell.