The Ancestry link and sync system has been very patchy since the May "upgrade", and as well as frequent orange and red sync status, I've occasionally got a "not connected" error, so if your problem is intermittent, it could just be that.
To answer your second question, as far as I know there is no way to recreate the link between an FTM tree and an Ancestry tree, once you've unlinked them (apart from just restoring the FTM tree from a backup taken before it was unlinked, of course). The only way to create a link is either to upload from FTM to create a new Ancestry tree, or to download from Ancestry to a create a new FTM tree.
To answer your second question, as far as I know there is no way to recreate the link between an FTM tree and an Ancestry tree, once you've unlinked them (apart from just restoring the FTM tree from a backup taken before it was unlinked, of course). The only way to create a link is either to upload from FTM to create a new Ancestry tree, or to download from Ancestry to a create a new FTM tree.