Problem

When i upgraded Prestashop 1.5 to 1.6.0.11 the product and category SEF / Friendly URLs do not work but the CMS pages do.

Cause

In my case i believe it was because i had custom Friendly URLs set so this solution might for everyone, in which case use the Useful Links.

Solution

1. Go to your prestashop admin / Preferences / SEO & URLs page
2. Navigate to the Schema of URLs section
3. Click on Save. 
4. Disable Friendly URLs, click save in this section, then re-enable the Friendly URLs
5. Test, You might get away with the steps so far.
6. As a further step, copy the Schema of URLs from a fresh install of prestashop (same version) and then click save.
7. Disable Friendly URLs, click save in this section, then re-enable the Friendly URLs
8. Test

Other Things to Try

There are several tutorials out there that tell you to experiment by turning the apache multiviews on/off and the same with mod_security. After you change either of these you need to disable the freiendly urls and then re-enable them, this causes the .htaccess to be regenerated correctely. These methods will only help a few people and you really should not disable mod_security.

Useful Links

• [SOLVED] If friendly urls enabled then product images do not show - OUT - 1.6.0.11 [7 Jan 2015] - PrestaShop
• PrestaShop 1.5/1.6 SEO and URLs Preference Options Tutorial
• apache - SEF URLs i PS 1.6 returns 404 - Stack Overflow
• Friendly URL not working - Ecommerce x PrestaShop - PrestaShop