The first thing you do is to book an appt with a solicitor. If your name is on the house deeds and the mortgage certificate, and you have lived / kept house there for 18 years, then the house is partly yours and you are entitled to some of the equity.
Take all the paperwork you can find, mortgage docs, bank statements, bills etc. to the appt.
And then stop worrying about it. Sit tight and wait until you have some legal advice, and you know what you are dealing with.