A default happens when a lender closes a credit account due to missed payments. At this point, the lender no longer sees you as a customer that will repay but a debtor that owes them money.
Defaults will stay on your file for six years. Unpaid defaults may result in legal action or debt repurchasing that can continue to impact your credit health beyond the 6 years.
There are free charities and organisations that can help with managing your finances to pay off defaults or speaking to your lender to come to an agreement. These include StepChange, Debt Advice Foundation, National Debtline and Financial Wellness Group.