What type of Backups have failed. Is this workflow running VM backups?

Also check the schedule of backups Ie full, incr and skip is set.

very likely due to having changed something in the settings of related to the workflow as the error messages clearly states "policy definition has changed"... so then you could no longer perform a restart but only a normal start.

We have take file system backup and also scheduled backup is full

This happens when you changed something on the policy (as stated in the error message)

So you cannot restart. 

But you can still "start"

copy the policy to a different workflow and try