Well, your player has done it; they forgot about their oath, what exactly it said, and now they broke it. While Oathbreakers are a fun paladin subclass inDungeons & Dragons,swapping subclasses by accident isn’t something the player might be interested in, and they may want to get their old oath back - or change it to a new one.
However, oaths are a complicated concept, as they revolve around the character as a person and what they stand for, meaning that they need to do something that speaks to their now-broken oath and uphold it once again in the hope of getting their powers back. How would that work for you, Dungeon Master?

8Confess Your Sins
Perfect For The Stereotypical Paladin
A simple concept that could be potentially solved without leading your player into a multiple-session quest is to confess their sins to a figure of authority, whether from the same order or a key figure in the city, and accept whatever punishment they need to go through for their crimes.
What exact punishment will happen is up to you, and it also depends on you and your player to be sure whether the character is being honest with their confession. But they can pay, be arrested for some time, do labor work, or this can even lead to another quest.

7Protect Or Save A Notable Figure
From Political To A Person Pure Of Heart
Saving the innocent is one of the noblest things you may do. If someone needs saving, whether it’s part of your campaign or just a bystander in the crossfire, saving this person is the perfect way to restore your oath.
You can focus on important figures for oaths such as Crown, or genuinely nice people for oaths such as Devotion. The only issue with this idea is that it could take a few sessions to solve the problem, but it is highly effective.

6Save A Whole Place
From A Group Of People To Cities Or Countries
Take the previous concept and raise the scale by a lot. Instead of saving a single target from a specific place or situation, put multiple people in danger and have your paladin fix that issue. Your campaign is likely going to have that in the plot anyway, so you might as well go for it.
You can even give your paladin extra credit if they put themselves in extreme danger just for the sake of doing the right thing, as a paladin should. It would even be poetic to kill them through a swarm of enemies only to restore them from death with their oath back.

Destroy Evil
Another great example of a quest that may or may not be properly tied to your story is to give them a particular target to hunt down. They can be an enemy of the paladin’s order, an evildoer hanging around the world doing their thing, a personification of evil such as a Demon, or even a group of bandits, a cult, or similar, and taking the whole thing down is the paladin’s mission.
This concept can also be developed into a bigger quest in case your player doesn’t mind being an Oathbreaker for a while. Lastly, depending on the oath and the personal issues between the paladin and the target, you’re able to evaluate them by taking the target down without being overly aggressive. It’s justice, not vengeance - unless it’s theoath of vengeance,of course.

Bring Light To Darkness
If you want to twist the previous concept, you can take the general idea of hunting a particularly evil person down, but rather than taking their life, the paladin’s goal can be saving them from their own darkness.
Not only do you get a fun quest out of it, but adding the dilemma of fixing an evil person and how to do so is a great and unique challenge in itself. You can also turn a potential charismatic villain into a future ally thanks to your paladin.

3Purify The Land
Save The World, Literally
Particularly interesting for restoring anoath of Ancients,though you can use it to any paladin, is to cleanse the darkness that befell the land. Purifying a location, whether it’s a simple house or establishment, to a good chunk of land itself is a fun and different type of quest to add.
This concept is also easier for you to control as a DM. To haunted house could be done in few sessions, while a whole region would be a much harder endeavor. And you have many ways you can go about it, such as destroying the source, roleplaying prayers and other methods, or finding a lost source of light, among others.

2Repair The Damage
Make Them Clean Their Mess
We mentioned earlier about confessing their crimes and accepting punishment, which is an effective way to restore your oath. However, if you want to take things in a way that feels more active to your player, you can have them find a way to fix the problem they created or at least dim the consequences.
If their oath-breaking cost an innocent’s life, they can either find a way to resurrect them or at least be financially responsible for burial rites and be there to make sure the soul will go to where it should be. Destroyed places can be repaired by the paladin, and criminals who fled because of them need to be re-captured by them, and so on.

1Obtain And Surrender Power
A True Test To Your Player
If you really want to test how invested your player is in their roleplaying, you can have them go on a quest to obtain something very powerful, like amagic weaponor something, and have them give it up to whoever sent them on the quest.
Having to obtain power to ultimately let it go is a ultimate test of character, meaning this is a good test in case you’re not sure whether the paladin learned their lesson or not. And, you can still let them have the item in the end if you want to, as long as they truly believe they have to give it up.