When a string breaks – a definite hazard in piano tuning – the simple answer is to replace it. However, the heavy, copper-spun bass strings are costly items because the best way to get a string that will blend in tonally is to have a new bespoke one specially made by a string-maker.
There are ready made ones available but they are only likely to be approximately the right diameter rather than an exact match.
A bespoke string is always better and the cost is not usually much more than a ready-made one. However, just sometimes a bass string will break leaving sufficient wire at the end to allow a new piece of wire to be tied on. Joining two pieces of thick wire with a reef knot requires two pairs of pliers, perseverance and a bit of skill particularly in getting the knot in the right place so it won’t snag on the bridge pins etc. but the end result is a perfectly matched string – and well worth a try. At worst it will only break again, then it would have to be replaced. These are a couple of my better efforts and they’ve lasted for years.