My key fob will not lock, unlock or start my car and I just replaced the battery yesterday. What is the problem? How do I reset the key fob?

Because you removed the fob battery and further compounded by the fact you disconnected the vehicle battery, the complete lock system and the anti-theft immobilizer system needs reprogramming. I am undecided whether a locksmith or an electrician is better, but would suspect the auto locksmith.

https://www.google.com/search?q=mobile+auto+locksmith+near+me

https://www.google.com/search?q=mobile+auto+electrician+near+me

The car should always be started with a proper chipped key. The valet key on my car has a chip in its credit card style case. It will start the car if the case is close, or you can give the key to someone just to gain entry to the car and not start it.

You need to visit an auto electrician, an alarm installer or an auto locksmith. The key fob needs re-pairing to the locking system.

Videos that might help

https://www.google.com/search?q=2012+Dodge+Charger.+Key+fob+not+locking+and+unlocking+doors+after+battery+change

..

It sounds like your anti theft has kicked in. You should call the nearest GM dealer and see if they have a technician you can pay to have come to you and reprogram your keys and fobs with their scanners.

If this only happens when the key fob is close enough for the car to "hear" the signals transmitted from it then chances are the fob is failing. The FOB will have to be taken apart, cleaned and inspected for contamination/corrosion/stress cracks.
If the car does this on it's own even when the FOB has no battery or is too far away to be heard, then the problem could be internal on the TWICE (Theft Warning Module) or a power lock switch.
Since you said the horn beeps I would say it is most likely a FOB inadvertently transmitting signals even when you're not pushing buttons. Repair or replace the fob and you should be okay.

you can try this, with the battery connected lock the car using the key fob, make sure no keys are in the ignition. Disconnect the battery again and let it sit for 30 min or so, reconnect the battery and using the key fob unlock the car, it should start, if not repeat the procedure and insert the key into the door lock to unlock the car, both of these solutions should reset the security system. Good luck.

The key fob is not getting detected by the bike.------- The turn signals start flashing because it thinks the the key fob is not close to the bike.------- The batteries are replaced, but still key fob is not getting detected.------ Try this :--- Turn the handlebars to the left and lock them with the key. Once locked, unlock and try to restart, this resets the alarm on my 2008 FLHX.---- Try raising the kickstand and let the bike go. And see if it runs.If it runs then kick stand switch needs to be inspected and replaced.------- -------- This will help.Thanks.Helpmech

once you have replaced the battery you must reset,unlock/hold key over 30 secs...then lock/hold key over 30 secs.....then press/hold unlock button on remote for 30 secs.....reset..

once you fitted new batteries to both fobs you needed to reset the car.....put key in drivers door and unlock/hold key over(windows go down)wait 30 secs then lock/hold over key,(windows go up)wait 30 secs,then press/hold unlock button for 30 secs.you have just reset the system.you have loads of other little gizmos but i will leave you to find them all good hunting....one side parking lights/driveway illumination/and more.
geoff

You should not have to reset the key fob after a battery replacement. The signal from the key fob stays the same and is programed to the reciever.
If the key fob needs to be reprogrammed then follow these steps.
1. Have all key fobs with you during programming.
2. Sit in the vehicle with all doors closed.
3. Lock all doors using the Lock/Unlock switch on the door.
4. Insert and withdraw the ignition key 7 to 9 times within 10 seconds (do not turn the key, just insert and withdraw).
5. The 4 way flasher indicator on the dash should flash once or twice.
6. Insert the ignition key and turn it on (key on, engine off).
7. Press any button on one remote and the 4 way flasher indicator on the dash should flash once.
8. If you have more key fobs to program use the Lock/Unlock button on the door to unlock the doors and then lock the doors.
9. Press any button on the next key fob and the 4 way flasher indicator on the dash should flash once.
10. Repeat if you have more key fobs. Only 5 can be programmed.
11. To exit, simply turn of the igniton and remove the ignition key.
12. Unlock the doors using the Lock/Unlock switch on the door and exit the vehicle.
13. Make sure the window is down when testing remotes in case you leave the keys inside.
14. Close all doors and test all remotes that were programmed.

Do not skip any steps or the process will not work.

If the 4 way flasher indicator does not work when inserting the key, you will need to start the process from the begining. If it still does not work the receiver may be your problem.

If the 4 way flasher indicator does not flash when pressing any button on the key fob then the key fob is the problem (dead battery or defective remote)

Had same trouble with my 406, fixing no problem, with new battery in fob, insert into ignition, turn key, BUT DO NOT START ENGINE, hold button for about 10 seconds release button and remove key from ignition, then try and see if doors lock on remote, if not try again, had to do this a couple of times, and it worked, sorry to say that now my key fob button has completely fell apart, through ware and tare