I mass deleted the products for a store, using phpmyadmin sql query.
Now there is a small issue, if i go to the back end Catalog>manage categories
There is no option to add anything, it just a blank white page, no nothing....
Also if i go to system>manage stores>StoreName there is no option for selecting any root category...
Any idea what has gone wrong and how to resolve it?
Hopefully you made database backup as you're going to need it. Magento database structure is so heavily cross-referenced that mucking around using queries is NOT recommended. Even top guns do it as a last resort, so restore database and either delete products from backend or use API like everyone else.