Sounds like a nightmare. Are you named jointly on the mortgage?
The questions you will probably be asked by a solicitor are:
Are you married?
Are you both on the deeds?
Is there a restriction registered against the property on the Land Registry? This means that if the other tries to sell or remortgage you will be notified (do they know your new address?).
Do you know if you are Joint Tenants or Tenants in common? The latter protects your share in the property.
Definitely ask for a mortgage holiday from the provider – they’re still giving them for three months, I think, might depend on the lender.
Definitely get some legal advice as soon as you can.