Veritable 8C no power?

I am a landlord and returned to the property after 10 years. The panel had no power to it, so swapped one of the internal fuses which then powered up, the panel alarm went off and the original code silenced it. The following day whilst the panel was unarmed, the bell box alarm went off and when I went to the panel there was no power so I was unable to disarm the bell box alarm. Non of the internal fuses looked to have blown or the main external fuses. After about half an hour the bell box sounder stopped. I just don't know where to start.