Click on or open File on the top menu bar and then on Exit to leave the program, and return to Windows™. A prompt will appear if you have a completed calculation that you have not saved, giving you a chance to save it.
Whilst highly experienced software developers use their best efforts to create stable programs, with literally millions of code instructions present in modern sophisticated computer programs, plus the added complication of hundreds of combinations of computer 'platform' and operating system, regional settings etc. with which the programs must be designed to operate, none can be guaranteed to be totally immune from 'crashing' or 'hanging'. A detailed description of key presses and operations that preceded any failure, plus any system failure reference numbers that followed the event, will help the developers to trace and perhaps eradicate the problem.