The last few months I’ve spend an absurd amount of time scanning family negatives. Over 12,000!
For this I used the excellent Nikon CoolScan LS-50, also known as the Nikon CoolScan V ED. These scanners perform superb, even though Nikon stopped producing them. Even support has been stopped or reduced. If you need one, eBay is the place to go.
In the process of scanning all these negatives, I ran into the problem that it had swallowed the film strip, and the entire scanner refused operation right after that.
This article shows you how you can remedy this with only a phillips screwdriver and some patience …
Please note that this article was written with the LS-50/V ED in mind, but it seems that other Nikon CoolScan models might be fixable in the same fashion (i.e. 4000 [IV] and 5000, and other models that look similar).