The only times I've ever had that happen have been when I reversed the motherboard connection on a home-built machine. If you recently worked inside your PC's case, double-check & see if you knocked any of the power connections loose & obviously if you disconnected them for some reason try reversing the HD activity wire.
Otherwise if you haven't been working inside your case & this happened 100% on its own, you likely have some kind of motherboard issue ... make sure you don't have excessive dust buildup inside your case which can cause all sorts of problems by getting some canned air & giving your machine a good cleaning.
There are other things to check out, but try these simple ones first.