Version: (using KDE 4.2.2) OS: Linux Installed from: Ubuntu Packages ... automatically, saying "kwin effects is too slow". After that it can be turn on again only after full reboot.
You can use the shortcut alt+shift+f12 to reenable compositing. You can also prevent kwin from acting like that (I have never experienced this behaviour) by going to "Systemsettings -> Desktop -> Desktop effects -> advanced tab" and check the option "Disable functionality checks" I think that the reason for disabling compositing after wakeup is caused by the change of time. *** This bug has been marked as a duplicate of bug 184607 ***
FYI: on my Kubuntu 9.04 alt+shift+f12 do not reenable compositing. As i say before, it reenables only after full reboot