I disconnected the battery on my 2003 honda accord ex and after fixing the battery the alarm system doesn't work.

SOURCE: HOnda Accord EX 1998 ALARM

Have you tried putting your Valet (original keys) into the ignition and turning it to the II position?

SOURCE: 2006 Honda Accord EX replace the battery now radio

Your car’s audio system will disable itself if it is disconnected from electrical power for any reason. To make it work again, you must enter a specific five-digit code in the Preset buttons. Because there are hundreds of number combinations possible from five digits, making the system work without knowing the exact code is nearly impossible.
You should have received a card with your manual(if you have one) that lists your audio system’s code number and serial number. If you don't have the card, you can get the code number from your Honda dealer, but to do this, you will need the system’s serial number, or maybe the accord's vin number.
You will see ‘‘Code’’ in the frequency display the next time you turn on the system. Use the Preset buttons to enter the five-digit code. If it is entered correctly, the radio will start playing.
This is probably the case. Hope this helps.

SOURCE: Automatic Seat Belt. in Honda Accord 91 EX

check fuses or see if something might have got jamed in the track and also try movin the part where the seat belt moves back and forth to see if the motor is dying

SOURCE: how to disconnect alarm system on 1996 honda

Disconnect the battery. Go under the steering wheel and look for a system of wires that is added on. Chances are that the alarm is taped were the wires are wrapped around the car wires and start removing. There will be a module to remove too. If you can't tell what is original and what was added then you can't proceed. Get a pro.

SOURCE: 1998 Honda Accord EX V6 2 Door - Radio Error Code on Display

No-one is ever particularly happy to here this, but this is the way it is; we do have to keep in mind that the original idea of codes on radios was to prevent people who steal them from being able to use them.

So, if an audio unit with theft protection is disconnected from electrical power, you must enter a unique code number into the unit or it will not work. This code and the audio unit’s serial number are listed on the anti-theft ID card that comes with the vehicle. If this card is lost, you must obtain the audio unit serial number in order to get the radio code (some people say that they have found the code on a sticker inside the glove compartment, but I wouldn’t count on that). Some newer vehicles allow you to retrieve the audio unit serial number electronically from the radio. Refer to your owner’s manual for the specific procedure (the following usually will work: Turn the radio off. Push the preset 1, preset 6, and power buttons at the same time, then quickly release. The serial number will come up in two sets of numbers.)
Some older vehicles (model year 2003 and older) require a visit to the dealer in order to retrieve the audio unit serial number (or you can remove the unit and get the serial number from the back of the radio). Once you have the audio unit serial number, you can call Auto Customer Service at (800) 999-1009 or have the dealer look up the radio code.

Here is the URL for the Honda Owner page. If you register (no charge) you are able to get radio codes using the VIN number of your car, the serial number of your radio, and the Phone Number and Zip Code that you give them when you register.

https://www.ahm-ownerlink.com/login.asp?page=%2Fprod%5Fhome%2Easp

Hope this helps you with the process.