How to close stale lightning channels with c-lightning
The problem seems that the other nodes are also offline which is why you can't connect to them.

Luckily you don't have to be connected  as you can do a force close by publishing your channel state to the Bitcoin network (the disadvantage is that you have to wait for the timelock until you can spend your outputs) 

Just try lightning-cli close ENTER-CHANNEL-ID-HERE in case you are not connected your node should perform a force close