When I do simple select or insert it says-
Unknown table engine InnoDB
It was working properly just few days back.what it's suddenly gone wrong ?
There are several reasons. Are you sure you've not changed anything (configs, updates?).
1) Make sure you don't have skip-innodb in your mysql file.
2) InnoDB might be used via plugin, this may be your situation. Connect to mysql, execlute SHOW PLUGINS and SHOW ENGINES. This would give some info. You might need to install innodb plugin(s):
You might also need to reinstall/recompile plugins if they were deleted for some reason.
Check your MySQL error log. If there are InnoDB related errors, the InnoDB table engine gets disabled.