Will not start

The flashing green light is the security system telling you that your key is not being recognized as a valid key. That can mean the chip in the key has failed (which is rare) or more likely that part of the security system that reads the key has failed. The fastest test if it's the key or the system is to use a second key to see if it works. If it does then the key is the issue and you would need to get a new key and have it programmed into the vehicle either by a dealer or a locksmith who does automotive work.

Because the light is working we know it isn't the fuse that powers the system that is the issue, the next most likely item is the immobilizer module itself. On that car replacement isn't real difficult as you remove the lower dash cover, then the steering column covers and finally remove the screws that hold the module in place and then the electrical connector. Then you reverse the process to put it back together. The nice thing about this system is that the key information is stored in the ECM so most of the time you don't need to reprogram in the keys as the replaced part only reads the key information and sends it to the ECM.