What is the problem when the car is just sitting overnight and there's no power on anything the next day?

check for fusible link in wiring harness

Before you ever disconnect any vehicle battery, you need
to have or know about anything that may have to reset.
Somethings you can do & some things need a professional scan
tool
I would say now is the time to gather the info & educate yourself,
as most vehicle are way more complex than a 2002
You really need factory service & diagnostic manuals, &
due to the fact almost no one works on their own cars anymore,
due to technology,that is expensive to buy,I have for 50 years
but that is my hobby & ongoing study
I would ask a tech at a dealership or use Google to search out
the question or solution to you current problem

Fuel Pump is a possible,check pressure
with a gauge, when you try & start it

Power On/Off and Key Lock/Unlock - Press the Power button located at the top right
of the Sky Caddie to power on/off or to lock/unlock the keypad. Upon pressing
the Power button you are prompted to choose either "Off" (Left Soft Key) to power
off the device or "Lock" (Right Soft Key) to lock the keypad. To unlock the keypad,
select 'Unlock" by pressing the Right Soft Key, then choose "Yes".

Go to auto zone and have them give you a code with the computer they will hook up.

check the bearings in the fan blade clutch
They can lock up at certain times & this did happen to me on two types of vehicles including a Ford truck

Take the SUV to an autoparts store that features free charging diagnostic tests. They will determine if the alternator is putting out sufficient power to recharge your battery. There's also a very good chance your battery is dead. It's not uncommon for a battery to need replacing (especially in a truck with all the bells and whistles I'm sure the Santa Fe has) after 4-5 years. It's also important to ensure there's no corrosion around the battery posts/cables (will have a white or green powdery appearance). Corrosion would prevent the battery from accepting a full charge.

I hope this clarifies your problem. It's unlikely the alarm system is the culprit provided it's not "going off" all day long. Best of luck and please feel free to post back with any other questions.
Greg

Check the battery for charge and alternator for power (free at Autozone) It might be deposited/rusted contacts on alternator and battery to reduce the electric current to battery from altrnator so battery just draining out. Also if it is very cold overnight (less than 0 F) battery lost the cranking power.
Please kindly let me know if the information provided was usefull for you or you need more info
Good Luck