Further to John Swan's previous post here is a photograph of the problem.
The explanation is that a small hydro-electric power station is being built on the West bank of the River Derwent and this will provide power to the Council House which is currently being re-furbished.
The blocked path is not a part of the NCN, but is (ie was) a useful link to NCN R6 which runs on the East bank of the river, from where the photo was taken.
In the photograph, A is the site of the proposed building, B is the blocked path, and C is the recently completed Riverlights development.
The Council House is just out of shot to the right of the picture, as is Exeter Bridge, which is the start of NCN Route 6.
Cycle route diversions have not been signed in spite of protestations from Derby's cyclists.
A suggested route via the bus station has been rejected, leaving the only other option as The Morledge, cross Traffic Street onto Siddals Road and re-join R6 at Five Arch Bridge. Not a route for the faint hearted!