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....