The Eye of Cthulhu disappeared after I reloaded the game. Is there a way to get it back?

I was playing Terraria earlier, having just reached the Hell layer (~3000ft.) and after I killed one of those annoying imps, I got a message stating:

You feel an evil presence is watching you.

I read on the official Terraria Online forums that it means the Eye of Cthulhu is about to spawn at the surface. Unfortunately, after receiving the message, I saved the game and quit to look online for that message means (while still in Hell). When I reloaded the game and spawned at the surface, during night-time, Cthulhu wasn't there.

Is there any way to get the Eye back?

It is possible to spawn the Eye of Cthulhu by using a Suspicious Looking Eye.

This item needs to be crafted from 6 lenses. Lenses are occasionally dropped by Demon Eyes (the flying eyes that attack at night along with zombies).

Once you have collected 6 lenses you will need to find a Demon Altar in order to craft the Suspicious Looking Eye. You will probably have to do a bit of searching underground in order to find one of these if you haven't already found one.

Note that in order to summon the Eye of Cthulhu it must be night time. If dawn breaks while fighting the Eye, it will fly away to return the next night with full health.

From the Terraria wiki page on the Eye of Cthulu

While playing, the player may get the message, "You feel an evil presence watching you...", which means that the Eye will awaken soon. After a few minutes the message "The Eye of Cthulhu has awoken!" will appear and the boss will promptly start to attack the player.

In my experience, you'll see the 'watching you' message about 3 times before the Eye of Cthulhu actually spawns. Just keep playing. He'll probably spawn in a couple days game-time.

The Eye should spawn a few minutes after you see the initial message. I've had the message appear 3 times so far and it spawned every time. I don't know if there is a health requirement, but if you are too far underground then the Eye will not spawn. Since you were in Hell at the time when you got the message, that's probably why you didn't it the first time.