Accidently deleted /var/vm, stuck on grey screen with apple logo.

Boot up holding command-r keys into Lion Recovery Volume.

Choose Re- install OS X.

Since you are running OS X Lion you can reinstall the OS using the Recovery HD volume. Reboot with Command-R held down to get to the recovery tools, and then select the option to reinstall OS X. This will take a while since the tools have to download ~3-4GB of installer files from apple, but will replace the missing components.

The second option is to restore from a recent Time Machine backup, if you have one, which would also be done through the Recover HD volume's tools. This is the faster of the two options, but will require you have a recent and complete Time Machine backup available.

I just checked and sure enough, /var/vm is backed up by TM

I am not sure this will help the OP, but if I use Go in the Finder and select /var and then run TM, I can see all the directories including vm.

I presume with TM connected one could simply restore the vm file rather than restore the entire OS, providing the restore allows this kind of selective repair.

Booting into a terminal and copying /var/vm back where it belongs might be all that is necessary.

Very true, you could enable the Finder to show hidden files and then restore just the vm folder with that.

The /var/vm folder is just used to store the sleep image and swap file for OS X, so you should be able to simply recreate it using the Terminal in the Recovery HD partition, and not have to deal with reinstalling or restoring.

To do this, select Terminal from the Utilities menu in the OS X Tools interface (when you boot to the Recovery HD partition). Then enter the following commands:

mount -uw /Volumes/Macintosh\ HD/

mkdir /Volumes/Macintosh\ HD/var/vm

After doing this, use Disk Utility to repair permissions, and then restarting should have OS X use the folder for the sleep image and swap files.

Message was edited by: Topher Kessler

Pure genius, Topher.😎

I wonder if the computer would start in the Safe mode, so that the file could be restored normally?

I don't know if vm is needed for a Safe mode boot. Just thinking out loud.

Hmm...this means your system does contain the /var/vm folder, so the problem is not because it is missing.

As for the permissions repair, you can try doing it using the Terminal in the Recoverh HD volume with the following command:

diskutil repairPermissions /Volumes/Macintosh\ HD/

If this does not work, then something else besides the configuration of the /var/vm folder is the cause of the problem here, and you would likely address it fastest by reinstalling OS X.

It looks like you hosed your system. If the permissions database is gone then when you ran whatever procedure that had you thinking you deleted the /var/vm folder, then you might have inadvertently erased a number of other files. While you might be able to restore them through meticulous work with the Terminal, a simple reinstall will repair the damage done.

Reinstalling will not wipe your disk, but instead will overwrite and replace system files on the drive. Your applications, user accounts, settings, and data will all be left alone, and the OS will be refreshed underneath it.