If the mortgage is in both your names, then your liability is 'joint and several'. That means that, irrespective of who is living in the house, the bank or building society can pursue either, or both, of you for their missing money. If they decide that you're the 'easy target' it might be you that ends up with a County Court Judgement against you and with no prospect of getting any credit.
Get some legal advice as soon as possible. In the first instance, your local Citizens' Advice Bureau might be able to assist but it's likely that you'll need to use a solicitor to force a sale.
Chris