If heads are stuck in the ramp then the drive was probably dropped, hard. They are then most likely bent, if you allow them to get to the platter it could be catastrophic. Also check for cracked/damage ramp. This is all speculation though, you need to take the heads out and inspect them under a microscope. If they are not bent you still need to make sure none of the sliders are missing. If any slider is missing then it's time to play hide-n-seek. Never power on a HDD, even after a head swap, if you can't account for every one of the original sliders.