Fatal error: Maximum execution time

My site was working find then it has now begun to get the below error - on home page and gallery pages. It seems to relate to pages that pull images from the media library. I also get this issue when I attempt to view the media tab, customise the theme and some other pages. I am unsure of how I can resolve this, any suggestions please. I have turned off sliders, removed all plugins etc and site is sort of working, but need to get it back up. Fatal error: Maximum execution time of 30 seconds exceeded in /home7/XYZ/public_html/wp-includes/functions.php on line 1425 Thanks in advance