Assuming you can get to the game options, try resetting the options to default (Game Options screen, bottom left). Also clear the PSO cache (Game Options > Graphics > PSO Cache).
After this, start a new game to check that all works. (Without mods. You said you have none, so you are good here)
As a last resort if nothing works, you can backup your saved games, textures etc, and try a complete reinstall. Then copy over your saved games.
I had listed the procedure for a reinstall in another topic:
Clean reinstall (Topic: How can I completely get rid of the Mods )