Re: Blank screen after upgrade
did you perform the db upgrade? (I guess yes, but you didn't report that in your message)
Ensure you followed all the steps reported here, just in case: upgrade2to3 using 3.1 directly.
Other options: try cleaning manually the temporal folders (./tmp/cache, ./templates_c , ./modules/cache ).
And in case you were using a highly customized theme, try removing the record related to your theme style selection (search for *.css in tiki_preferences table at your mysql db) so that it uses the default theme style.
Finally, check that your host meets the Requirements for tiki 3: php5, mainly, afaik.
HTH. And please, report back when you solve it....