So the battery is ok.

Lot's of nasty little things that could be wrong, so let's try and work through them.

Check the battery leads.  The terminal posts should be clean and the leads attached firmly.  Make sure the earth lead is attached to the frame, I've known very strange faults due to dodgy earth lead connections.

 If the ignition light doesn't come on - check the fuses, if fitted.

Does the ignition switch work?

Has some joker turned off your "kill switch"?

 Has the bulb blown in the ignition warning light?  This wouldn't necessarily prevent the engine from starting, but on "battery sense" type alternators ( Lucas ACR range ) it will stop the alternator from working.