You are lucky! Formatting won’t make the data erased permanently. Those deleted data is still stored in the usb drive.It’s just invisible or inaccessible.
The most effective way is to rely on some data recovery programs. Try this program called Tenorshare Data Recovery. It can recover data after deeply scanning your usb drive to look for lost data you accidentally deleted or formatted from various data storage devices(like USB Drive, Pen Drive, digital camera card, memory card, SD cards, floppy disk,hard drive,etc).
It is important that you stop using the usb drive. The less new data is written, the better the chance of data intact recovery.
Just have a try!Hope it helps! Check it out: http://www.any-data-recovery.com/product/index.htm