How to undo "Erase" from iCloud's Find My iPhone?

Hello,

I lost my iPhone4 last night, the person who stole it switched it off right away. I logged into iCloud and activated Lost mode THEN Erase. I found out later that when I Erase the phone, which will happen once the phone is connected to the internet, I will not be able to track it via iCloud or do anything anymore.

Now the phone is still switched off, how can I "undo erase" so it doesn't erase when it's switched on? Thanks.