Check that your engine is free - its best to use a spanner to turn the crankshaft pulley nut. Seizures are very uncommon, but possible! If engine is free to turn, then your starter simply cannot do its job because 1) The battery hasn't got a full charge 2) The starter motor is duff 3) the heavy wire connection to the starter from the battery is slack or dirty 4) The black wire from the battery to the bodywork is the same 5) The engine will have an 'earth strap' somewhere that connects the engine metal to the bodywork that may be again, loose, dirty or missing. You need to check all of these things before the fault will be revealed. A motor factor will be happy to test your old battery for performance too - and even happier to sell you a new one if necessary!
John, February 2008