Hi there,
it's not much of information you give. It could be loads of things causing this fault. My guess: faulty K1/2-overvoltage protection relay. It's the sleak silver one with the white plastic top and two fuses in it. It's located next to your battery. We had scores of them replaced due to a cold soulder and even today I see one every now and then. Touch it and let somebody turn the key to pos. II. You should feel it clicking every time you turn the key. If you catch it while the car isn't starting you got it.
All the best,
Dokta