When you say the collet has worked fine for years... Do you mean this specific collet, or the collet closer itself?
Either way, take your spindle liner out, clean the whole thing carefully, make sure the taper doesn't have any dings or rough surfaces.
If you mean one specific collet no longer opens as it should, chuck it, it's worn out and won't spring open anymore.
If you mean the closer itself, and now NO collets will open properly, then get a new spring, it's worn out and it doesn't matter how stiff it "feels" to you.