My external alarm bells (2 at high level)are not sounding when the system is activated/tripped.The internal bell on the panel are activated.

Could the bells have been deactivated via the keyboard?

If so could you please tell me how can they be reactivated.

I had understood that if wires to alarm bell were cut the internal alarm would be activated.

If the wires to your bell are cut, and a SAB is fitted, then the external bell will sound when the wires are cut, but external cabling to the bell should be inside the building or protected in conduit

It may be that you have programmed a bell delay.