Assuming it is a protection diode in parallel with the battery? (protection diodes can be either series or parallel - and series reduces battery life), then it's just a simple rectifier diode, any small rectifier (1A or so?) would do as a replacement. Also, simply removing the S/C diode would make the meter work again, but would give no reverse battery protection.
As for why it's failed, it's because the battery has been connected the wrong way round, even if only very briefly.