My car alarm kept on going off all night so i disconnected the battery. When I reconnected it today in the monring, the key wouldn't turn, thus, can not start the car?????

Fob didn't work so the alarm immobiliser was armed and active when the locksmith opened the car - with predictable results.

Fob still not working - broken or needs new battery. Have you fitted a new battery into the fob?

The alarm immobiliser will probably remain armed until the doors have been locked and then unlocked with the fob.

There is a small chance the system can be fooled into thinking it has been disarmed by disconnecting the battery (overnight if poss) and then with the door open and the ignition key in and turned on, when the battery is reconnected it might be possible to start the engine...

try sticking the key in the door

Temporarily disconnect the horn and consult a Professional key programmer.

During a blizzard i lost my keys (with beeper) in the snow and only had my spare key in the house so when i opened my car the alarm kept going off every few seconds, after a lot of searching i found a solution:

If it is factory alarm simply disconnect the negative terminal of your battery and put your key into the ignition and turn it to the ON position, then reconnect the negative battery terminal and start car, alarm should be off and reset. worked for me :) hope this helps others in my situation.

The alarm system somehow got activated , this is usually because the battery becomes discharged or disconnected try doing this:
Remove the key from the ignition
disconnect the negative battery terminal
put the key back in and turn to ON
exit the truck and leave door open
reconnect the battery terminal
get in and leave door open, if the alarm horn stays off it should start.

Disconnect the battery then put key on and reconnect the battery. As soo as the battery is hooked up start the car by turning the key forward. Do not turn it back. After it is running disconnect the alarm

do you have a remote door opener fob,try locking and unlocking doors with it

Before you disable your alarm try this, disconnect the alternator field plug over night, check battery voltage at the battery before you disconnect the plug then disconnect and leave alone over night, make sure you have at least 12 volts when you disconnect the alt.
Recheck battery voltage in the morning before you reconnect the field plug...

I suspect you may have a leaking diode in your alternator's internal regulator, have seen it many times on your model... Certainly worth a try. Let me know how it turns out....

i took the car to a mechanic and had the alarm system disconnected and the horn reconnected

If this is a factory alarm all you have to do to turn it off is put your key in the door ,lock it then unlock it . if its not a factory alarm you will need the remote or have the entire alarm unhooked by a mechanic