In my Ubuntu 10.04 installation I twice experienced this problem:
I installed Picasa on wine and it was installed and running perfectly for some time.
Then I experienced when I started Picasa that apparently it started but was never shown on screen.
I checked that the process "Picasa3.exe" was on the list of processes, the status was "sleeping" and "pipe_wait".
When I log on at other users of my Ubuntu installation, Picasa was running like it is supposed to do.
How can I solve this problem?
Remove the folder ~/.google/picasa and start the application again, this will reset your Picasa settings and cache.
Please be aware that you will loose you pictures/images library so a backup seems in order.
To make a backup and remove the folder from your home open a terminal and type
mv ~/.google/picasa ~/picasa_bak
You will be able to find your previous settings and library in ~/picasa_bak. In case you want to restore the old folder just move it back to where it was before.
You can also do this by opening Nautilus and in your home folder press Ctrl+h, look for the .google folder and move the picasa folder somewhere else.
Also, if you think this might be a bug, you can report it on the Picasa user groups here
