There are a couple of reasons why you have the problem.
First of all, before you put any form of heating on you must ensure the tank is full.
To check if water is actually going into the heater, having said none comes out of the tap, remove the drain bung behind the external cowl and operate the pump, water should come out quite quickly.
If it does not, then the non return valve which is situated in the cold water inlet fitting on the heater has stuck and will need replacing, or the inlet waterway is blocked with limescale.
In either case it is advisable to get a mobile engineer to sort it out as the Cascade is an old heater and needs very careful handling to avoid damaging it beyond repair.
The various fittings are very brittle and will almost certainly snap when being removed and it is a delicate job to remove the broken bits that stay in the heater, also if the tank has to be separated to clear limescale then in all likelihood the tie bar will snap and need replacing , and getting the broken bit of tie bar out is very very delicate and needs special attention to avoid damage that will render the whole unit scrap.