The key cylinder on the driver door broke. I took the door apart and replaced the cylinder with a new key. Now my door is locked and I can't unlock it and if I open the other door the car alarm goes off and I can't start the car. I put the key in and as soon as I turn the key to start the alarm goes off again. How do I reset the alarm to start the car and unlock the door? It is a 1998 VW beetle.

Sue we arn't talking keyless entrey here .
Then if the key works in the other locks it must be good not worn out .
Witch leaves the lock cylinder on the drivers door or the mechinizam that moves the rods inside of the door to lock and unlock it .
Dose the key feel like it fits ok and the lock turn normally .If so that leaves the mechinizam inside the door .
Its not the end of the world likely something popped off or a little plastic clip broke that holds the rod in place .
The door needs to be taken apart from the inside witch means it needs to be open to repair .Its like a caught 22 can't fix it unless its open but can't open it .
There are ways slim jim or the old coat hanger way if you can roll the window down think about it do a little reserach it can be opened with out distroying the door

Use a Slim Jim to pop the door open or have AAA do it for you. Once open take apart the interior side of the door to gain access to the door lock mechanism and reattach the control rods for the lock with new clips.

If the key won't turn in the lock cylinder a locksmith can replace the parts by taking the door apart. Sometimes a worn key won't turn the cylinder on one side and a new key can be made by the key code. A locksmith has the codes and tools.

Does it have a chip in the old key? If it does, try holding the chip next to the key and start it, see if it unlocks the steering. Get a dental pick and pull that busted key out of there. That is weird about the steering, I thought it was a mechanical lock, how did the key break? Oh, I see, you took out the cylinder, you didn't have to do that. That is why the steering doesn't work, the cylinder unlocked the steering, Clear as mud now. Get a new key in there, you should be all set.

my 97 had the same problem. if the key fits the lock and will turn the lock but the lock wont open the door then your problem lies in the linkage between the door lock cylinder and the locking mechanism.

if the key fits the lock and the lock wont turn at all, this is what happened to me, then your lock is seized. Ive actually seen this on alot of 95-99 sentras it seems to be some sort of design flaw. what I had to do was to replace the lock cylinder and locate a new key. the locks from a dealer are a little pricy so i went around to salvage yards, every nissan sentra from said year range door locks were all seized, I even tried the 200sx as its the same car but a 2 door, same problem. luckily i found out that the 94 nissan altimas have the same door locks so I dismantled the inside door panel of said altima and snagged both front door lock cylinders. i also got lucky as the key was still in the ignition, replaced both locks and got a key for about 20 bucks. depending on your problem you might need new lock cylinders

It sounds like the actuator for the lock has seized up and may need to be replaced...

Below are instructions for changing the lock cylinder as well as a few diagrams. Best of luck.
Greg

1. Disconnect the negative battery cable.
2. Remove the lock cylinder bezel from the steering column shroud by carefully prying with a thin bladed screwdriver.
3. Remove the shroud fasteners and remove the steering column upper and lower shroud panels.
4. Insert the key into the ignition lock cylinder and rotate to the RUN position. Then rotate the ignition cylinder back to the ACC position.
5. Depress the locking button on the side of ignition module assembly with a thin pick-type tool. Slide the lock cylinder assembly from the ignition module.

INSTALLATION:

1. Insert the key into the ignition lock cylinder and install into the ignition module.
2. Install the steering column upper and lower shroud panels and the shroud fasteners.
3. Install the lock cylinder bezel.

Reconnect the negative battery cable.

Quite possibly the linkage was not connected to the new door lock handle and key lock. Will be necessary to remove door trim to reinstall

according to my partsman (who has had great advice for many years), you do not have to remove the door panel to replace the door handle which contains the cylinder lock. the handle is released with a screw which you can see when you open the door. you will need a starhead screwdriver or socket. A new handle costs around $40 at a parts store in my area. call around for a locksmith who will re-key your key to the new lock or use the key which comes with the new lock, your choice. i am in dealing with this also

you have made a boo boo somewhere,are you a certified city and guilds technician,if not then you shouldnt be repairing customers cars