A good way of finding out answers to problems is to Read The Manual (RTM for short [or in extreme cases RTFM]) that comes with whatever program you are using. If you open the manual and do a search for "spacebar" it's amazing how the answer magically appears.
Just this once, because I'm feeling generous, I would suggest opening File/Settings/Program/Playback/Program Behavior and alter it there.
Small correction to the where the setting is - File/Settings, Program, Playback, Spacebar Behaviour.
I too would advise you to read the PDF manual available under Help - you may have to download it - all basic setup items and usage of the program are covered in the manual.
If the spacebar goes back to the very beginning, there is a problem with the project. It can be resolved by one of two methods; use the second one if the first one doesn't work:
Click on the double arrow (arrows pointing outwards) at the bottom right of the Project Window;
Ctrl+A, Ctrl+C, open a second movie, Ctrl+V, delete the first movie, Save as - with a new name/revision code.