No heirs = you're screwed. Next time, take better care of them.

Well, take better care of the *good* heirs. If you have an inbred, cowardly, unhinged loon heir who would be the first in line to the throne, get him killed any way you can.

Actually, you have one way out of this mess: start a civil war and back the rebels. Raise taxes to maximum, strip your least loyal generals of their titles, get yourself excommunicated, lose some battles, destroy loyalty-increasing buildings, move troops out of disloyal provinces... do everything wrong, basically. And if you're lucky you'll get a civil war.