Blank page, but can Administer it

Hi all,
after a disaster on my windows server I reinstalled evrything (php5,apache2,mysql) from scratch on an ubuntu OS. I exported/imported my old 5.x drupal site to a new 5.9 site, importing all the old modules and performing the required steps. It looks fine if I navigate in the Administration pages and in the menu content links.
But if I try to go the the root page I get a blank (well, white) screen. I also get blank pages in some other contexts (access to taxonomies and blogs).

I guess my current install could lack some extensions and deffinitely must be revised, but...do you have a quick fix for that? It would help me a lot to have the initial page working at least.

Thanks in advance!
Josep M