If you are saying the engine sounds like it is at 5000 rpms or 600 rpms
without any input from you?  Or, is the tach broken and reading wrong.
If the car is running at the rpms you state the cable has to be stuck
and the throttle won't release.  And, you should not start it again
until you figure out what is causing the high rpms or you are going to
do serious engine damage.