Battery Drain leads to Big Mistake.

[gidpor]

We had not used the car for two weeks and when I attempted to yesterday, it would not start. I figured I must have a power drain somewhere because the battery should be good. Nevertheless the key opened the driver door only, not all four as usual. The instrument lights came on when I turned the key, the starter clicked, but no go.
I had it jumped and it fired instantly. Drove it around for a bit and parked for the night.
- First oversight on my part was not to disconnect the battery, so In the morning I was faced with same issue - Instrument lights come on- turning the key gets a few clicks but no go. Now I knew for sure there is a power drain somewhere.
I went for the jumper cables again thinking I really need to get it charged this time.
Just before getting in the car to give it a go, I noticed the the positive end of the jumper cable was not seated properly on the Battery terminal - Without thinking I reached for it, disconnected and repositioned getting a huge spark in the process. (what was I thinking being so reckless !!)

What have I done ? -
No instrument lights come on when I turn the key, and no clicking sound at all.Real dead. The battery now reads less than 6volts across terminals.The OVP fuse seems good - Still I replaced it with a backup with no effect.
Where do I need to look (for a blown fuse ?) TIA

126 - 300SE 1987 Euro