This is a question that I am having trouble answering for my campaign. I am going to be running a creature that is able to charm and control other creature's actions for a long period of time. If a Paladin, Cleric, or Warlock has their will taken over by an Evil creature and are forced to do actions that go against their oath, god, or patron, would the character lose access to any of their features or spells?

In general, no. If your campaign world has a stipulation that clerics and paladins get their spells from gods then presumably the gods, as higher beings, are intelligent enough to distinguish that the oath has been broken involuntarily.

That being said, in your world the gods may be ***** so who knows :)

Right, generally no. RAI for warlocks is that patrons give them class features and levels, but they are not so easily taken back. Paladins might change their oath under extreme circumstances if they feel they have personally broken it. Gods are usually pretty busy and don't pay too close attention to their clerics, but if one were to act in a way that directly opposes the God's goals (within their portfolio, not just alignment), they would notice. Clerics might be the only ones at risk of losing class features.

Clerics and warlocks can level up again by getting a new patron/God.

In any case, I wouldn't recommend doing anything without talking to the player first. It can be a great roleplay/character growth moment with cooperation.

Not all patrons (or gods) are opposed to a bit of evil if you catch my meaning. And how do we know that the period of being charmed wasn’t all part of that gods grand plan? As for oaths, it’s kinda impossible to keep a promise to be an incarnation of an ideal when one’s free will is stolen. The multiverse grants exceptions for stuff like that generally.

Do you intend to have your players be charmed for long periods of time? Doesn't sound like a great time for the players?

This was for an NPC Paladin in my campaign, and I felt like I should ask about warlocks and clerics to see how this would affect their magic.

Ah ok, sorry if I read too much into it. Hope you found your answers.

I feel that most deities are lenient towards Clerics and Paladins that perform actions beyond their control. Warlock Patrons don't fuel their Warlock's powers. Rather, they reveal to them magical secrets and abilities, so a Warlock's power they can keep, even if they entirely break their pact.

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This is a question that I am having trouble answering for my campaign. I am going to be running a creature that is able to charm and control other creature's actions for a long period of time. If a Paladin, Cleric, or Warlock has their will taken over by an Evil creature and are forced to do actions that go against their oath, god, or patron, would the character lose access to any of their features or spells?

In ascending order of issues:

Warlocks: Samus is correct about their power source.  Warlocks aren't powered by patrons in the first place, so it's a non-issue.  I'll also note that under ordinary circumstances, pacts can't be broken.  You can't just decide not to give up your soul after you've sold it.

Clerics:  Clerics are directly powered by deities, but a deity's opinion of this situation will vary by deity.  A competent or compassionate deity will understand that the cleric isn't choosing these behaviours, and will hold the real party at fault accountable.  A petty, stupid deity might blame the cleric, despite knowing better.  If you're asking about an NPC Cleric, not a PC one, decide how you want your deities to act.

Paladins:  This is a lot like Clerics, but the judgment is rendered by a mortal - the Paladin themselves.  Paladins are powered by their own confidence - their strong belief that they're right.  Anything that can disrupt or shake that belief can potentially threaten their class powers, as you see fit (for an NPC) - including feeling guilty for falling victim to mind control magic.  The more self-doubt they feel, the "weaker" they get, conceptually.

To sum it up, all that stuff is fluff. Pacts and divine powers have no rules attached to them. A base assumption in 5e is that every PC is entitled to the features they have gained during character progression. You need to make houserules if you want certain behaviors to actually remove class features.

It's generally considered more appropriate to have story consequences instead. Maybe a cleric needs to do penance. Maybe a warlock is forced to show their loyalty through a quest. And if the player resists, they are encouraged to rethink their future character progression to better reflect who they want to be.

Of course with NPCs, do whatever you want. They are not PCs and don't have to follow the same rules. If it makes a compelling story, go for it.