if it is an incompatible mod move the mods folder to the desktop then clear the cache files launch and see if all is well if it is go check your mod/s are the same patch level as your game so for example if your game is 1.67 and a mod is way below that then yeah you will have issues hope i make sense here then place the mods folder back BUT open that mods folder and cut/paste your mods files to the desktop place one mod back launch and play if all is well that mod is fine then the next one if it fails delete it from your mods and clear the cache file again rinse repeat
to delete the cach files go to electronic arts/the sims 3/
•CasPartCache.package
•compositorCache.package
•scriptCache.package
•simCompositorCache.package
•socialCache.package.package
it's safe to delete these files as a new ones will regenerate
also if you have in your saves file folder any files with ,bad delete them as they are no good and will keep causing issues AND the game can be set back to factory settings without having to uninstall and reinstall everything just make sure to back up your stuff before hand
http://sims3.crinrict.com/en/2011/06/tutorial-resetting-the-game-to-factory-settings-without-reinstalling.html