2003 Nissan sentra check engine light flashing and car won't start?

A flashing check engine light (CEL) means it needs to be taken to a mechanic now. Nissans have a history of sensors coming on, but a flashing sensor indicates a problem requiring immediate attention. Not sure why the car won't start, but could be a number of reasons. I had a fuel sensor go bad on my Maxima and it wouldn't start because it flooded the pistons, so I replaced that sensor. But you may have a different problem. Bottom line, have it towed to a mechanic.