My websites have vanished in Dreamweaver 2021

You must define your sites in new Dreamweaver.  Either from previously exported STE files or by manually creating them.  Site > New Site...

any chance for STE files ?

By any chance, did you export your sites (STE from any version of DW) or synchronize Dreamweaver with the Cloud (only if between CS4 and 2021 you had an intermediate version)?

Alternatively if this is the same computer and you recently upgraded or you have access to the original computer, you can still grab the files directly without DW: 

Mac --> Library:Application Support:Adobe:Common:{Version}:Sites:Site Prefs

Win --> Registery --> HKEY_CURRENT_USER\Software\Adobe\Common\{Version}\Sites

If you still have the older version of the program installed, and your site files are on your local machine, you can open the old version and go to Site > Manage Sites > Export to create .ste files for your site settings. Close the old version and open the new one, then go to Site > Manage Sites > Import to bring the .STE settings over.

If you don't have access to the older version, but you do have your site files saved locally, you will need to create new sites in DW under Site > New... then use the existing site folders for the Local Root Directory and re-enter your FTP information manually.

If you don't have the old version of the program, and you don't have the files saved locally, you will need to create a new, blank site with an empty root directory on your hard drive. You then enter the FTP information and save. Open the Files window and choose the Get option (down arrow icon) to download a copy of your site from your server.