The OP isn't very informative on what happened to the full extend.

I can't remember having read which windows version he uses. This could be vital info.
If it's any recent version with standard setup of primary partition C:\ he might use "recent version" feature for the Bitcoin folder to extract any older/recent version of wallet.dat, even if it got deleted. I would try that first (in the non-destructive way). But read next paragraph first and obey the advise.

But the very first step to do is to make as early as possible a complete image backup of the Windows partition which holds the vital data.
It depends on how many Bitcoins might be in danger, but I'd do that even for less than one Bitcoin. The basic idea is to be able to recover a known starting state before tinkering with the system while trying to fix the issue. If you do something wrong, breath smoothly, and recover your old state to start again.

The OP said that his Bitcoin client started to reload the blockchain. (We still don't know which version of the Bitcoin client the OP uses. You get the point? Pretty scarce information policy by the OP... why that? Do I smell something fishy? Strange that a lot of other posters didn't mind to ask obvious questions...). I have no idea why the Bitcoin client felt the need to reload the blockchain, but again valuable information isn't available to judge. (OK, OP might not be expert enough to know what is important and what isn't.)
The walled.dat might be just fine. As long as the blockchain data hasn't been downloaded, I guess, the Bitcoin client won't show much to be in the wallet.dat (I might be wrong here, as it could flag the Bitcoins in the (old) wallet.dat as still unconfirmed.)
As others said, normal client won't touch the data directory in any harmful way.

I won't emphasize too much on the unspeakable stupidity to not have any backups, especially when substantial amounts of Bitcoins are in question. So, I guess you don't care much about the data on your hard drive which could fail at any time, btw. I have seen hard drives passing away, though not completely by surprise. Well, it happens here and there...

I hope you can recover your lost data and I wish you learn your lesson to show more respect to your valuable data goods on your hard drive. Not all data deserve it, Bitcoins surely do!