You have probably got insufficient voltage over the jump leads for the ECU to permit the engine to start cranking. The ECU is almost certainly waiting until it has 12.5 volts or so.

Best bet would be to remove the battery, charge it properly overnight, and replace it. THen you'll have full voltage and the ECU will have reset.

Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE