You question may imply two completely different possibilities.
1. Windows machine converted.
Did the VM start? Probably not, but if it did, you are done. Besides, converter has te possibility to just configure a Windows VM, Try it, sometimes it works even if it has failed as part of the conversion. If it doesn't - upload the logs bundle for examination.
2. Linux machine.
You may try manual reconfiguration after booting from a live ISO image - preferably the installation CD of that distro and version. Install GRUB, create the initramfs image, check and fix the fstab and network config files. Uploading the logs may be useful too, to diagnose which part has failed and eventually why.