I have an issue with the lock on the driver's door on my s5.
The key doesn't go all the way in the whole and I can't unlock the car.
Please help me if you know what and how needs to be done to fix it, thank you.
I tried to open the plastic cover but cannot reach the lock from the inside due to the window being in the way.

 

The whole handle assembly must be removed to get at the lock cylinder.
Take the lock cylinder to a locksmith and have it rebuilt.

 

Roll up the window so you can access and remove two 10MM bolts with a 10mm socket with extension. There is a little plastic cover on the door to allow access to one of the bolts. Move the window up or down as needed to give best access.

Remove the three philips screws that hold the locking mechanism to the door jamb. This is needed to allow "wiggle" to work the door handle off.

Reach in and push off the swinging connector for the rod that runs from the door lock mechanism to the door handle.

From the outside of the car, wiggle the handle out from the top then toward the front of the car until the handle frame just comes out. Gently pry the swinging snap off the large door rod then pull the handle frame from the rod.

Pry off the circle clip for the locking cylinder and the cylinder simply push's out.

Then, as already advised, take it to a locksmith and have it cleaned and oiled.

What probably happened is the little plastic flap broke and jammed up the keyhole blocking you from pushing the key all the way in. The flap is what minimizes water and debris getting into the cylinder. The locksmith will be quite challenged finding a replacement flap, so you'll probably end up with an open keyhole.

Have fun

 

I have an open keyhole on my drivers side and let me tell you... Its almost impossible to wash my car during the winter because as soon as water gets in there it freezes. I broke my main key trying to open it.

 

chew gum.
put on key hole.
wash car.

 

Chew car
put on gumhole
car wash

 

Thank you very much for the step-by-step instructions for removig the lock. I was able to remove it and take it to a locksmith.
He turned out to be pretty good. After 2 hours he had it fixed. He even put a new cover with a working shutter for $19 all together.
Now the thing is if I'll be able to put it back together and have it work properly.
I did drop a nut and possibly one more misc part into a very nasty place between 2 metal parts on the door. I hope it was just the nut as I can replace that but I thought I saw something else there too.

 

HOZZMANRX7's write up should be stickyed. He nailed all the points.