Assuming it is in joint names, the person who moved out can apply for a court order to force the one still living there to sell.
Unless there are exceptional circumstances the court will usually grant the order as it is recognised that it is unfair that one of the owners can neither realise his share of the property to buy or rent another home whilst not getting any benefit from the home.
Exceptional circumstances apply where children are involved and they still live in the home, in which case the order to sell may be postponed until the youngest is 18. This is in no way guaranteed, though. If there is enough equity in the property to allow the parent who looks after the children to buy a suitable home then the order will be made.
Profound disability may be another exceptional circumstance - for example, if the house has been specifically adapted to meet the needs of a disabled child.