If you're unable to repay a personal loan that you took out with a cosigner, both your and your cosigner's credit scores can be impacted. Because both of you are legally responsible for the repayment of the loan, your lender can require that either you or your cosigner pay.
If your co-applicant can afford it, they may have to make those payments until the loan is paid off. If they're unable to repay the loan, you both may default on the loan. In this case, your loan may be sent to a debt collector that may attempt to collect it or sue you both.