One hopes that there is always some storage space that is not in use.

If space runs out in Main Memory, the computer will crash, that is, stop working.

There are programs that sense when space is getting short and warn the user. The user could then close some of the open applications to free up more space in Main Memory. Sometimes the warning is too late to prevent the crash. Remember that all the data in Main Memory vanishes when the power goes off. Thus a crash can mean a lot of lost work.