Sims 3 will no longer run in full screen mode. Why?

Did you update to 1.69 ? The launcher should give you the version in the lower right corner.

Your system seem to have a dual graphics card. Make sure it runs on the Nvidia one and not the Intel: http://sims4.crinrict.com/en/2014/09/graphics-cardintel-chip-dual-systems/

The article is for Sims 4 but just do the same for Sims 3 (or ts3.exe)