I'm feeling lost and uncertain about my job.

I've been working with my employer for almost 20 years, initially as an executive assistant. However, when my VP retired, instead of letting me go, they demoted me to an admin role for several departments, each not under the same umbrella of VPs. I find myself doing very little work for my supervisor and her departments, mostly handling purchase orders and invoices. Over the past few years, my supervisor has been insisting that I don't have enough work (Red Flag), even though I believe I am just efficient. Despite my efforts to seek work in other departments, my supervisor has been reassigning my purchase orders to others who are not admins, and these individuals are not informing me when they receive my purchase order against an invoice. I have talked to my supervisor about her redistributing my work while claiming that I don't have enough work, but I only receive evasive responses. I can see what she is doing, but I'm not sure how to stop her. I haven't approached HR about this issue as I'm not sure if this is something I should take to HR. I had hoped to retire from this company, but no other open positions were available. Should I go to HR, and if so, what should I tell them? I feel my boss is not giving me enough work, and I fear my job is at risk. It might sound silly, but it's quite frightening for me. I feel like I will complain myself out of a job.