@evan@e14n.com power and vol - for fastboot mode. If that works (depends on how standard the boot loader is) you may be able to use the 'fastboot' command to re-flash it to stock. (If you can get a copy of stock firmware).

Once,on the fastboot screen you can use up/down to cycle through the various options. If there is a recovery option you can try that and (depending on the recovery firmware installed) either wipe the device or if a good recovery do all kinds of useful things. If bootloader (fastboot mode) supports the fastboot command from a PC you can flash a good recovery firmware (if one exists for the Xoom) and then boot into the good recovery firmware and use it or adb to fix the system