If you are amenable to using the console, generally you still need to find where your follower died to ressurrect them: when you do, open the console, target their corpse, and type: resurrect 1 The only thing you can do at this point-if you're not keen on loading up an earlier save or using console commands-is to find where your follower died and loot their corpse.
You can confirm this by trying to pick up another follower: if they agree to come with you, the other follower has ceased to be. If fast traveling still doesn't produce your follower, they're almost certainly fully dead. If there's a chance your follower is still alive but nowhere to be seen (for example, if they entered the pre-death state and you ran off, thinking they were dead dead), fast traveling to a location should cause the follower to appear back at your side. However, if they continue to take more damage after entering that state, they will die permanently. When your follower 'dies', they enter a pre-death state that generally allows them to come back to full health immediately after battle.
