Deja vu. There have been a few posts on this. This is exactly what happened to me... drove my new purchase home and parked it in the driveway, only to find out it would start next time. Hooking up a temporary push-button switch between solenoid and battery (actually, 12v terminal at starter) got me running and into the garage.
In American cars, cleaning the battery terminals has fixed this for me more often than not. In 914s, I've had more problems with ignition switches.
Quick and dirty check: Stick a screwdriver on the hot terminal of the starter and tap the tip against the solenoid terminal (make sure car is in neutral of course - if you aren't comfortable with this, say so and I'll post photos)... does it engage/crank? If not, you have a bad ground, power source, solenoid, or starter (in order of likelihood, IMO). If it does crank, you probably have a bad ignition switch, or possibly bad connection in the circuit between the solenoid and ignition switch (particularly if you have the seatbelt interrupt junk under the passenger seat).
For me it was a bad/cracked ignition switch, which is pretty common with 914s in my experience.
This article may help, or post follow-up questions.