In the Lightning Network, what happens if a "middle node" suddenly goes dormant for a long while?
If the transaction cannot resolve before the CLTV value of the HTLC runs out the peers will be refunded (the pending HTLCs cancelled) and the transaction will be cancelled.
If C is not there to receive R funds will not move so there is no need for force closures, but the funds will be stuck on the online channels as well for that period of time.
The online channels will be able to propagate payments in the meantime given there is more liquidity available and the number of pending HTLC-s is below the set limit.
See the situation where C would be online to receive R and go offline before propagating to B discussed in:
What happens if R is not revealed by one of Lightning Network nodes? (Payment cancelation)