Fatal error: Cannot redeclare wp_nav_menu()

Have you tried:
– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– If you can’t get into your admin dashboard, try resetting the plugins folder by FTP. Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– switching to the unedited default Theme (Twenty Sixteen, etc.) for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins). If you don’t have access to your admin area, use FTP , or your web-host’s cPanel or whatever file management application your host provides (no Dashboard access required). Navigate to /wp-content/themes/ and switch to the default theme by renaming your current theme’s folder by adding “-old” to the end of the folder name. Alternately, you can remove other themes except the default theme. That will force your site to use it.

Hi, I try what u said and checked other websites and post for info related to the problem, but i got tired after some days and deleted all files related to wordpress in the website, the pluggins didnt show and when I try to re install the wordpress plugin it was like there was no site on it, even if i saved the contentwp forlder or whatever is the name, I still got the “backup” of the page that was before but i never got to see it. I contacted the company who made that website and they couldnt fix it… they just said it was pluggins error and that was easy to fix but when they try they couldnt fix it…, since Im just learning bout wordpress lets see what the future brings. thanks for the help !.