It sounds like what happened to me. I installed "The Dude" v3.6 service on my 750 and it locked up. I ended up using NetInstall to boot the device and disable/uninstall that package. You can use it as kind of an emergency boot disk to see if you device at least boots. From there you can reload firmware if needed.
If you made a backup. You can easily try to just set it to factory default and reload your backup file. Just hold the reset button until the light next to it starts flashing. Then restore your backup. If you don't have one you'll at least get more practice at configuration.
Netinstall goes one step further. You install it, then follow the instructions for net booting your device. You can see if its at least capable of starting.