HI guys

I got a server Dell Poweredge 6800 , with a Raid 5, which the client ran chkdsk on it and after booting windows gave him a blue screen and he proceeded to reinstalling the OS to his surprise all the data was gone , has anyone has ever experienced this? and if yes can we get the data.

Obviously he just reformatted the existing raid set, so a first guess would that standard tools for recovering (quick-)formatted disks would do the job. If he would do the best to his server and formatted it with complete overwriting or re-arranged the disks in the raid set, he should be prepared to bite his own ass.
Why do people with less than home user knowledge have access to a server?!

Because mostly they don't pay for the damage, "Mr. Jones will fix it!"
Anyway if it was a RAID5 and the OS was fully reinstalled the original RAID structure was overwritten, block size etc. It is a tough job. Every drive should be analyzed and the whole RAID set in turn. Anyway don't panic : I was able to reconstruct a destroyed and partailly overwritten RAID but it was an awful job and it did cost a lot of $ to the customer. In my case one of the drives failed and the customer tried to fix the drive while keeping it on the RAID set. Not a clever idea.

P.S. "he should be prepared to bite his own ass" ... LOL... we say he should be prepared to eat his own... er... family jewels in case of male customers. (a bit of humour to point out the differences in saying the same thing around the world)