This is two separate phenomenon

1. Death of the quirk

This is where the quirk naturally slowly disappears with constant use. This does not typically harm a user unless they rely on or are only alive due to the quirk.
The quirk can not be reactivated.

2. Death of user

This is death of the user due to the quirk. In this situation an "undead" version of the quirk is activated.
This version of the quirk is the only reason the user is alive. The quirk may also fly out of control and many who experience this phenomenon are likely to go into hiding.
It is reported an abandoned city in the mountains was populated by these people and the oldest was 100,000 years old. Though specifics are unclear as the researcher swore to secrecy and soon after disappeared.