Like he said, put a jumper across the starter terminals.

With the key on, the car should crank right over and the jumper across the terminals.

My 1986 740 Turbo Wagon had a very similar problem back about 5 years ago. Mine was intermittent.

Do the windows go up/down ?
Do the central door locks work at all ?
Do you get power anywhere when you turn the key to the ON position ?

If the key turns on and you don't get any power anywhere, start looking into the IGNITION SWITCH in the steering column.

I would not be surpirised if your problem turned out to be the ignition switch.

Also, if there absolutley NO POWER under any condition, I would suspect a bare battery cable - usually grounded against the frame or motor somewhere. I had that problem with one of my old BMW E21s about 12 years ago.
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'92 740 B230F Wagon::'92 B230F 240::'86 B230FT Wagon (RIP-Parts Car)