It can be very stressful when you have been given Notice of Sale. The first thing you should do is to stay calm and do not delay your response, when you have been notified.
You should contact your lawyer and provide any documentation you have received from the lender and their lawyer. Ideally you want to see if lender may consider stopping power of sale actions, by paying all arrears and costs and carrying on with your mortgage. If they do not want to work with you, contact a mortgage professional that can act quickly on this matter.