Cache folder isn’t writeable
– Messed up backend, and the sliders doesn't show up in the frontend.
– "Failed to open stream: Permission denied" error message with a file.
If you press F12, click on Console, refresh the page, where you have an error with the Smart Slider, and you see this problem:
Then your cache folder might be not writable. Smart Slider is using 4 folders in the wp-content/cache folder on WordPress and media/nextend/cache folder on Joomla. The four folders are css, image, js, less, so you should check out them, if they have 775 or 755 permissions, or on some servers 777 is needed. You can do that for example with FileZilla, if you connect to your ftp, look for these folders, right click on these and click on File permissions…
Change the Numeric value to 777, and check the Recurse into subdirectories.If this doesn't help, try to give another cache folder at the WordPress' Settings → Nextend Settings → Cache Path. If your backend looks messed up, do it like this.
If only the 777 permission works, but you don't want to use that, check out this documentation.