If you don't pay a collection agency the collection agency will continue to call you and send you letters. The debt collector will harass you to no end.
Eventually if you don't respond the debt collector may decide to sue you in court. If you don't show up for court then then debt collector automatically wins the case.
If you're sued by a debt collector you should show up in court and plead your case.
A debt owed and put on your credit report will stay on your credit report and affect your credit score for 7 years.
After 7 years if the debt collector doesn't try to sue you then they will be unable to sue you due to the statue of limitation.