Normally the original supplier will offer a guarantee ( normally 5-10 years), if there is no guarantee then,
there are 2 main options either:
Replace with new ones or
If leaded or bevel designs on the glass try having them 'split & reboxed' (reusing existing glass but with new spacer tube and re-sealed) from a Double Glazed Unit manufacturer.
Mike at Agree-a-price frames, September 2010