Without knowing exactly what you have done with input files, how can I
possibly diagnose your error?
Make sure you are using the latest version of FDS.
Check carefully that you are following the instructions in the users
guide for doing a restart.
If you are doing both of those and still have a problem open an issue,
post the simplest form of your input files that results in your
problem (i.e should run quickly and containing the bare minimum inputs
to have the problem occur and not a 10,000 line input file that takes
days to run).