complexity of finding a cycle with total weight k in a graph when you know no hamiltonian cycle in graph satisfies the given weight k
I don't know about knapsack problem, but it isn't too hard to reduce from Hamiltonian Cycle to this problem (assuming you mean a simple cycle). We start with an arbitrary graph $G$ with $n$ nodes for which we want to find a Hamiltonian cycle. Then we construct $G&#39;$, which is $G$ with the addition of a single node $v$, a single edge from $v$ to an arbitrary node in $G$ with arbitrary weight, and every other edge has weight 1. We then ask if there is a cycle with weight $n$ in $G&#39;$.

If we have a Hamiltonian cycle in $G$, this corresponds to a cycle in $G&#39;$ with weight $n$. This is not a Hamiltonian cycle in $G&#39;$, since it doesn't go through our added vertex $v$. If we have a (simple) cycle with weight $n$ in $G&#39;$, then it must include exactly n nodes. It can't include $v$ since it has degree 1, and can't be part of a cycle. Therefore, this goes through all the nodes in $G$, and thus is a Hamiltonian Cycle in $G$.

Therefore this problem is $\mathcal{NP}$-Complete, unless I misinterpreted your question.