Hello.
"What Happens to the Data if the System Craeshes?" - Sometimes everything is fine and you can continue working with the project. Sometimes the project can be damaged.

"Is the Any Option to Retrieve The Lost Data?" - Maybe. If you have a backup copy of the project. For example, look at "Documents \ Incomedia \ Website X5 - Pro \ name of your project \ Backup. " Or try to send the project to the company's employees. Perhaps they will be able to help you restore it.

Hello,
as mentioned by Aleksej, this depends, mostly on the timing of a crash.
If a crash occurs while a large modification is ongoing, or during the save process, there may be an increased likelihood of project corruption.
In general the recommendation is to make frequent backups with the function Export the project, so that you can store recent IWZIP files on an external storage in case the main project has Issues.

Eric