What could be wrong??

There's just too many possibilities and it will be moot to guess at it.... unfortunately that's all ANYBODY here can do!... The best route to take, is have it scanned with a professional scanner to find the culprit... That don't help get you going right now but it's about the only way to successfully diagnose the no start problem without throwing a bunch of parts at it, which tends to be more expensive...and still may not fix the problem! ... **Has the timing belt been replaced?**

Try making a new ground cable from neg on batt to tranny or engine