If you access the operating system on the server where this happens through Remote Desktop and open Task Manager, do you see any process for Excel? (Be sure to look under the Details tab). If you kill that process, does that resolve the situation?
I have never used the ACE provider myself, but all through the years I've been monitoring SQL forums, there have been no end of posts where people have problems using the ACE engine. I can't escape the feeling that it is not a tenable solution. But then again, I never see posts from sites where everything is working OK.
Anyway, in Object Explorer: Server Objects->Linked Servers->Providers find the the provider, check "Allow inprocess" and restart the instance. It may help for this problem. But it could also create new ones. If Excel crashes, this could now bring the entire instance down.