Immobilizer sign comes on?

[berksman1]

hi in frontera today and as dead as could be nothing worked at all. took battery off and charged for half hour and started up straight away now the immobilizer sign has come on over the petrol gauge and nothing but all the lights and clicking? does this mean it all now needs resetting or a new immobilizer ? thanks. left one the battery to see if drained and then would be alternator but due to this now i cant find out. easy fix or thoughts please?