Are you sure you didn't just download a new server and run that?
If so, then the reason is because you are running a fresh server and it could mean that your old server is just saved somewhere else
Make sure you have a good look to see if you can find where it used to be and if you do find it, find the folder entitled 'world'
Make sure to keep a copy of these files somewhere else in case
Then find the folders which contain the new server you downloaded
Delete the folder entitled 'world' in the new one and copy the one you grabbed from the older server into the server files
if you are unable to find the old server then have a look in your recycle bin (this should be found on your desktop)
If you can't find your old server, it may have been deleted completely but if i remember correctly files that are deleted are never actually deleted
They are just stored somewhere and files can be saved over it so try and find a program that could find this.