I once started my group of players off at level 45. However, they did not know that they were level 45. I created their characters for them, but I gave them a sheet with level 1 stats on it but NO class information. So naturally they thought they were level 1 and had no idea what class they were playing.
Before we even started playing... I had determined that their characters were searching for a super rare and power artifact that an elemental lord on demi-plane held in his possession. They fought, and he was able to throw the characters from his plane back into the material plane. Only they were badly beaten up I decided. They fall from the sky into the ocean where nearby villagers went out and pulled their bodies from the sea. When they awake they are all hanging upside down by their feet over the mouth of a volcano, with hundreds of villagers chanting nearby. They are the new sacrifice.
I decided that the characters had a bit of amnesia (to make it fun). I'd have them make Intelligence checks once in awhile to see what they remembered. (They didn't know why I was having them roll it). As the villagers were dancing about the roar of a loud and angry red epic dragon could be heard somewhere on the island. The villagers fled in terror leaving the characters hanging over the volcano.
My players tried all kinds of ways of getting free, some worked, some failed. But as time went on, they gradually started to remember who they were, through successful Intelligence checks. Once they were free, I gave them character sheets that said they were level 12, with level 12 stats and abilities. They were surprised, but happy.
Long story short, they discovered that Fire Giants lived on the island, and for some odd reason there was an army of them moving toward them, still a bit away though.
The characters panicked, and I allowed them to make one final roll. That is when they discovered that they were in fact level 45. They couldn't believe it. I've never seen players so happy. It was like christmas.
If anyone is wondering, their epic gear was stripped off by the villagers when they found them. (This made it easier to decieve my players).