Hi
first of all collect the notice
depending on the demand and allegation reply to that
take the notice to a lawyer and get the advise depending on your decision of divorce
Most likely the notice may be demanding divorce and alimony, and other reliefs, so if you do not want to continue you can agree for a settlement and finish it
if in case the notice is allegations which you prefer to reply reply to that and see what next step has to take.
consult a local lawyer with the notice.
There is nothing much can be done by panicking, take it as it is and fight it .if the relationship has reached to standstill end it amicably and start afresh.