First step is to try your spare key. If the theft light is flashing with the key you've always used, it can only be one of two things. Either there is a problem with the chip in the key or there is a problem with the PATS system. There is no way to start it with any kind of reset. If it doesn't start with your spare key, 90% of the time it's the PATS transponder which receives the code from the key. There will be codes stored in the PCM related to this problem. Have you changed anything on the car just before this started happening??? Oh, the odd time to many keys on a key ring can cause problems. Try with the key alone, not likely the problem but eliminate the easy things first.