Rooted Htc one m8 TWRP recovery stuck on skydragon installer

Do you have just root or root and S off? Does your firmware match the ROM version?
Hopefully you made a backup, if not Sprint devices are easy to fix.
Since you unlocked the bootloader you might have adb and fastboot setup unless you used a toolkit....
If you do have a working adb/fastboot folder you could use adb reboot recovery.
Unplugging the phone and holding power until it shuts off or a battery pull should work as well.

 

Battery pull's going to be a bit difficult on an M8.

Before trying to flash SD you did run a full nandroid backup in TWRP didn't you?

What version of TWRP do you have?

If you press and hold power plus vol up for about 10 secs it should force a restart. Once screen goes fully black, keep power pressed and hold vol down to boot to the bootloader.

In bootloader select hboot (vol keys to move select and power to select), wait until hboot loads and then select recovery. This will get you back in to TWRP, from where you can restore your back up, or try reinstalling SD - but I'd forget about selecting any options for Xposed until you have a successful install first.