You can try CTRL-C yes.
This will safely shutdown OpenRefine and save your project locally to the workspace folder. (If it doesn't close the terminal window automatically after a few minutes, then I'm afraid that something wrong has happened during the export and causes OpenRefine to completely hang)
You can then start OpenRefine again and open the project again and try to export in a different format if you wish... perhaps only selecting a few columns into CSV file, if it is a really big dataset as you suggest. Then save the next set of selected columns of your choosing from the Export dialog into CSV again. Then finally you can load those CSV files into Excel or LibreOffice, etc.
Let us know if that helps or if you have further questions.