If the other server crashed unrecoverably, you will have to seize (you can’t transfer it anymore) the role from a surviving DC.
Check here on how it’s done:
Using Ntdsutil.exe to transfer or seize FSMO roles to a domain controller
After that, clean out your AD from the leftovers of the dead DC:
How to remove data in Active Directory after an unsuccessful domain controller demotion
Directory services restore mode dcpromo /forceremoval