404 after installation of 2.8, site down

the previouse wp installation ran great, tried to upgrade to 2.8, cause the old one was a bit slow. but now 404. all blockend. no better with getting back with the backup copys. i tried as well to wipe all on the ftp and just install everything from scratch..no luck either..still 404. even installing the whole stuff in a subdirectory gets me a 404. if someone can help me out, ypull be blessed…cheers bob