Have you been using any cheats or mods?
Because normally you do start the game with Force, and Force "restores itself" over time (be patient).
It shouldn't just run out and never come back, unless you somehow cheated to make yourself a level 0 Jedi or something.
Note: on the first couple of levels (at the Jedi Academy) you DON'T start with Force. If you're on those levels, see the walkthrough... because they are finishable:
http://strategy.jediknight.net/jka/contents.shtml)