The Tip of Your Tongue
The word is there. The path to it isn't.
You know the word. You can almost feel it, maybe you know the first letter, or how many syllables it has, but it stubbornly refuses to surface. This tip-of-the-tongue state reveals that memory isn't one system, it's two. Storage is where information is kept. Retrieval is the path your brain takes to reach it. When a word is stuck, the memory is intact, but the path to it is temporarily blocked, often because a similar-sounding word is getting in the way. That's why you can sense the word's shape without being able to produce it.
In real life...
First, don't worry, this happens to everyone, in every language and at every age. When a word is stuck, try running through the alphabet or describing related concepts out loud to jog retrieval. If that doesn't work, let it go. Your brain often continues the search unconsciously, which is why the word tends to surface later when you've stopped trying.