i am not able to enable compiz . it was working fine previously .

[srikanth_vm@localhost ~]$ compiz
compiz (core) - Error: Screen 0 on display ":0.0" already has a window manager;try using the --replace option to replace the current window manager.
compiz (core) - Fatal: No manageable screens found on display :0
[srikanth_vm@localhost ~]$ compiz --replace
compiz (core) - Fatal: glXCreateContext failed
compiz (core) - Error: Failed to manage screen: 0
compiz (core) - Fatal: No manageable screens found on display :0
[srikanth_vm@localhost ~]$

i have disabled other window managers ,how to enable it again ?

i recently did an update , could this be the problem ?
i am using fc 10

If your update included a kernel revision you also need the matching mod for the graphics. It sometimes takes a few days before it appears as an upgrade at which point your higher functions in terms of graphics will start working again.

This is one of the downsides to using bleeding edge software. I learned from the same experience to not apply new kernels until I saw the matching Nvidia update as well.

Fixed the problem :-)
the problem was with the kernel update , i downloaded driver from nvidia site and installed it again
Now everything is working fine