Retrieval failure is a problem with retrieving/recalling the memory. This may be because, whilst the memory is in our long term memory, we have forgotten the information (cues/prompts) that makes us think of the memory (cue-dependent forgetting).
The three kind of cues that you can have for a memory are context-dependent cues, state-dependent cues, and category/organisational-dependent cues.
Context-dependent forgetting occurs when our external cues at the time of encoding do not match those present at recall.
State-dependent forgetting occurs when our internal cues at the time of encoding do not match those present at recall.