There's basically not a lot you can do at this point.
Your first port of call would be to boot into Recovery mode by holding CmdR during startup. From there, you can open Disk Utility, and try to verify permissions. Note though that the permissions changed by this procedure do not incldue all permissions on your system. So, that will probably not help.
Through recovery, you can reinstall OS X from scratch without needing a disc or flash drive containing the OS – it'll simply download everything from the Internet. See this Apple support article for more. This will not remove your user's files but only replace system files. Chances are good that after the 4GB download and installation, you can at least boot into your system again. You could then run a Repair Permissions from Disk Utility again.
That, or restore from your Time Machine backups, of which I'm sure you've made one, right?