Maybe you try to start Apache twice?? As a service and manually or similar? So that the second try is already blocked by the first instance?
Anyway, there *should* be an error message, which you still not applied, which should give some proper hints. Either in the error_log and or on the screen (or Xampp Controlbox). Last not there may an issue with a firewall or so. It is extremely hard to find the error with this poor amount of information.