When the engine won't run:
Is there fuel at the injectors?
Is there a strong spark at the plugs?
Is the cold start injector switching off properly?
Is the distributor cap clean and dry inside? (Damp would usually cause a rough idle misfire, but I had one cut out after a mile once and it wouldn't re-start for twenty minutes).
While unlikely, it could be worth borrowing a known good fuel pump relay, just in case.