Joomla 2.5 error – Can’t add or edit articles and categories at back end by kyriakos4

I can not access some option in back end (administrator) in Joomla in “article Manager” “Category Manager” and also the “Global Configuration”. Each time I’m clicking on “New Article” or to any “excisting… (Budget: €30-€250 EUR, Jobs: Joomla, PHP)