Partion dissapeared on my removable drive

I have a 300GB drive in an external USB 2.0 enclosure. Two partions, 40GB and 260GB. The second partion (260GB) is somehow gone. When I look at the drive in Disk Management in Windows XP, the 260 GB appears as unalocated space. What happened to my partition? Any way to recover the data?

 

first of all you need to find out the reason of the loss of the partition.
to recover lost or accidentally partitions you can try Recovery Expert, read more here

if it would help to recover data you can use getDataBack for fat/ntfs, here