Yes, I know the location where the wallet is installed. But what file must I use to open the wallet again? There I find amongst other files the file default_wallet. But this file has no extension and Windows 7 does not know how to open it. Moreover also in the control panel I don't see the program.
Maybe the installation was corrupt?
The directory for all wallets is typically
C:\Users\YOURNAME\AppData\Roaming\Electrum\wallets.
You can open the wallets only using Electrum. Therefore, please open Electrum and it asks standardly which wallet you want to use, then click on 'Choose' and select the wallet. After that, click on next.
If it opens directly to your wallet, please click on 'File' and click on 'Open' in the menu, it asks what I wrote on the above.