Hi Jean-Philippe;
The best solution for this situation to always have a full back-up of your entire PS server so that you can just restore the entire environment should the server ever have a non-recoverable crash.
The other alternative is to just back-up the PS license file named "license.appeon" that is normally located (for example in the .Net version of PS) in the "C:\inetpub\wwwroot\appeon\AEM\bin" folder. This second alternative has more work though in restoring the entire environment as you would need to reinstall IIS, PS, copy the back-up of the PS license file to the above mentioned folder. Then ... re-deploy all your PS Apps again.
HTH
Regards ... Chris