Most moulds have a long but limited life due to cracks appearing and accidents. However the life of moulds can be extended with repairs. Most moulds can be repaired, unless shattered.
Cracks can often simply be ignored. If the glass is not getting marked by the crack, then you can keep using it until it widens or goes completely across the mould. If you feel the need to protect the mould before it completely fails, you can add a layer of cement on the back of the mould to support it.
The cement can be a high temperature product like “Sairset” or any other high temperature ceramic cement. The one I like is cement fondu. It comes as a powder – often from sculptural suppliers – which you mix with water to a paste. Wet the mould well to ensure it does not pull the water out of the cement, causing it to fail. Then apply the cement liberally to the back of the mould over the crack.
If you feel the need, you can fill the crack from the front also. Again insure the mould is wet and then press the cement into the crack. Wipe the excess cement off immediately or it will stick leaving blemishes on the mould. Use a wet cloth to do this. You can smooth the filler by using a wet finger to run along the filled crack. These notes apply to which ever kind of cement you use.
Divots or little chips from the surface of the mould can be ignored, if there is no effect on the glass at your operating temperatures. If they need to be filled, you can use a temporary patch by making a paste of batt/kiln wash and smoothing it over the divot. This will last a couple of firings probably. A more permanent repair is to use cements. Prepare as above and smooth into the depression. When cured, particular attention will need to be paid to getting a good coating of batt wash, because the cement surface will reject the water carrying the powder more than the ceramic surface does.
If the mould has broken you will need to stick it all back together. Do not attempt to smooth the edges, they are needed to make as close a match as possible to each other. The rough edges provide a key to location as well. Soak the mould pieces very well. Prepare the cement and apply a little to one edge of the matching pieces. Press together firmly and then apply a backing of the cement as for a crack. Clean off the face of the mould with a wet sponge or cloth until it is smooth and level with the working surface of the mould. Bind this as tightly as the shape permits and leave for several days.
When using refractory cements, it is best if you can give it a wet cure for a day. This is often easiest to achieve by putting the cemented mould in a plastic bag. After the one day wet cure, it needs to dry for several days. Finally, it needs to have a permanent cure by firing to a temperature of about 25C above the operating temperature for the mould.