94 318i starts then immediately dies

Sunday afternoon, my 94 318i started then immediately powered off - within a second or
two, everything electric appeared to be dead, no power.

Turning the key did not restart it. No response to key on/off.

I re-tried several hours later, & got the same response.
Starts then and everything turns off.
Just before the retry I did a lock unlock in case it was EWS related,
central locking worked. After it died again i I tried the driver's door lock,
and only the driver's door locked & unlocked no central locking.

Not sure this is related but:
I had just done a rear break job & put in new shocks Friday
I drove about 100 miles with a few stops & restarts Saturday (amazing how much difference
new shocks can make), no issues.
Have not yet reinstalled the trunk liner after removing it to gain access to the shock towers.

Any ideas what to check?
I should have early version EWS anti theft immobilizer. Could this be kicking in? Main
relay failing?