Who takes over when the personal representative of a will has died?

You question does not indicate whether the Testator has died or not. If the Testator is alive and able to execute a new will or codicil, then the Testator can choose a new PR. Typically, a well drawn will indicates a successor PR if the primary PR is unable or unwilling to serve as PR. If the Testator has died then it is up to the Orphan's Court to appoint a named Alternate PR or a new PR in accordance with the law setting forth who has the priority right to be named.