My MacBook no longer connects to my iMac via screen sharing
Things that have helped me with isolating the failure and eventually getting screen sharing working again:


isolate the network - turn off
WiFi and connect the two machines
with one ethernet cable. This
eliminates ports (other than
internal firewalls), DNS, and
network issues from mis-routing the
traffic.
Turn off screen sharing and remote management and set it up again changing the password to be sure the new password and sharing permissions are set for all users or your specific user.
Watch the logs in console app - the bonjour daemons, screen sharing daemons and network errors are all logged so you might get a clue if a program is having problems accessing a keychain or other issue
Add in a new user to isolate the desired user's preferences and files - it could be something corrupt in the desired user and you won't catch that unless you make a new account to isolate that potential cause.