my 97 c180 is not starting?

i beleive theres a fult with the imobiliser , when i turn the plastic key in the ignition, the ignition lights come up, but the starter motor does not operate , i have tryed to toe start the car, and i have checked the starter motor and i know it works, i have tryed leaving the key in the igniton for 90 minutes and it does not work also i have tried to hold the lock and unlock button on the keys for 5 seconds and it also does not work , i have checked all the fuses in the engine and the boot compartment all to know avail.