I have a Yardman walk behind mower model 18M. Recently the blade fell off while mowing. When I checked, I found the blade and the blade bell support but the bolt was missing. I figured it had just come loose and I hadn't tightened it properly the last time I replaced it. But when I went to replace the blade (actually a new one) with a new bolt, I found that the bolt won't fit into the blade adapter. And the adapter is firmly attached to the mower, as if the bolt was still there. So I'm thinking that the old bolt must've sheared off.

How can I get it out? Or, since the mower is about 7yrs old, should I just junk it and go for a new one?