1997 bmw 740i loss of power?

was driving all nite long for 4 hours down a dark country road when my instrument cluster started to flicker on and off, and then about a mile up the road the car just died, everything was dead, had no power ,nothing was working ,my incar fone also shut down. was left stranded for hours. walked a few miles back and found a motel with a gas station, awaited tow truck next morning ,got car towed to garage ,couple hours later ,the mechanic drove my car back to the motel and said he couldnt find a fault with it..says car is running fine...seems to me something weird happend that night..