I accidentally deleted index folder on my workstation, how to retrieve it?

It depends on what kind of workstation you are using, and what you mean by "delete". For example, in Windows, the default "delete" action is to move files to the "recycle bin". If the contents of this special folder have not been emptied, you just need to open it, select your folder, and then select 'restore".

But on other operating systems, or if you explicitly requested it, the "delete" action may not have a simple or reliable recovery mechanism. In that case you will need a 3rd-party "undelete" tool. These can usually be found online for sale and a few might even be free (or free to try). But be aware that successful recovery is not guaranteed.