What forensic psychologists do when victims have no recollection of memory (when crime happened)?
Memory loss is fairly common after a traumatic event.
Referring to events such as sexual abuse or rape - we refer to memory loss or amnesia as 'psychogenic amnesia' or sometimes as 'dissociative amnesia' in events such as those mentioned above.
(Mechanic et al 1998) did a review of rape victims and testimony and found that:
"Memory deficits for parts of the rape were common 2 weeks postassault (37%) but improved over the 3-month window studied (16% still partially amnesic)."
Where psychogenic amnesia is found, often as Mechanic et al found, time is a factor, and memory may improve at a later date. Often gradual recall takes place. Harsh interview practices with sustained questioning should probably be avoided as this wouldn't help much.
Interestingly Dissociative Amnesia is similar and refers to the blocking out of events due to the stress suffered.
Recommendations for dissociative stress include CBT and psychotherapy. The exact treatment will very much depend on the unique character of the dissociation however and is tailored to the specific type of dissociation occurring.
This article is useful: https://my.clevelandclinic.org/health/diseases/9789-dissociative-amnesia