My '97 did that to me once. If you don't normally use the immobiliser, switch it on for a minute (ignition off), then switch off and try again. Mine started. The immobiliser doesn't prevent the starter from turning.
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '11 MINI Countryman Cooper D All4 '90 LR 90 SW