I tried unlocking my car with the keyfob it wouldnt work. So I opened it with the key the alarm went off and a button saying off just below the stereo wont stop flashing. The car wont start now and to top it all I have just discovered that the key will only lock the drivers side door not the whole car.
In most vehicles of any year you have to have the Electronic Remote Keyfob to reset the alarm system once it has gone off. You can try to dis-connect the battery, wait one minute and re-connect. However, you need the code for your radio or it will not work. Your car may not need one, but since the Security sytem was tripped if it does it will be needed. If you bought the car new they would have given it to you. When you re-connect the battery, the radio will display, "Code" or, "Enter Code". You will have to talk to a dealership about resetting and retrieving the code if you do not have it as it requires a special tool.
To get the key fob to woek, hpefully it only needs a new key. You hjave to carefully pry it apart to replace battery. It is almost worth going to a dealership for this as well becasue if there is a way to do it withotu needing to have it re-programmed, they will know. Sometimes replacing the battery, and if the keyfob needs replacement you will have to, requires that the keyfob require a flash re-programming. This is very expensive.
So, I would go to a dealer and go to the parts Department and ask if they know how to replace it and if it will require re-programming. I say the parts department becasue you may find someone nice enough to take it apart for you if they know how and if you go straight to service, you will probaably get charged.