just looked into this a bit more. It is quite a fiddly area of law.
There are two ways in which the footpath could have been diverted (and from what you've said it seems to be a diversion rather than temporary closure).
It could either be under S257 of the Town and Country Planning Act 1990 or a temporary Traffic Regulation Order under S14 of the Road Traffic Regulation Act 1984.
If there are any notices up from the council it should confirm which route is being used.
If it is the S257 and the order has not yet been confirmed then you can make representations to the council. These basically need to be taken into account and if not withdrawn might lead to an inquiry (the process is more detailed than that but just trying to give you an idea).
If it is S14 then it appears the maximum initial period for a footpath diversion is 6 months - although this can be extended by the Secretary of State - pretty much indefinitely it seems. The maximum period for an initial highway diversion is 18 months though - so I am wondering if your footpath is technically a highway? Either way, if this is the route they have used you are looking at judicial review as your challenge - not cheap or easy. However, if it is a footpath and they have given an initial order for 18 months then you might have good grounds to challenge.
The Ramblers website has good notes on this.
www.ramblers.org.uk/advice/rights-of-way-law-in-england-and-wales/rights-of-way-and-development.aspx
And the Rights of Way Officer should be able to confirm the process to date, what the status of the footpath is and assuming s257 applies, the closing date for any objection.
If you are thinking about any legal routes of challenge (i.e. judicial review) you'll need to get RL legal advice asap.