Need to mount a snapshotted LUN or a LUN that was suddenly lost

You can't mount a snapshotted LUN and get immediate access to it.  You might in some rare occations also lose a LUN even though you're not dealing with snapshots. You might lose access to a LUN if a LUN is being accessed through different paths at the same time.

There are no obvious error messages in the logs, but on the console of the ESX server you will typically see a message like:
0:01:25:51.208 cpu6:1048)LVM: 4476: vml.02000d0000600a0b80000f229e000002554777b7
92313734322d39:1 may be snapshot: disabling access. See resignaturing section i
0:01:25:57.042 cpu6:1048)LVM: 4476: vml.02000d0000600a0b80000f229e000002554777b7
92313734322d39:1 may be snapshot: disabling access. See resignaturing section i
0:01:26:01.267 cpu3:1048)LVM: 4476: vml.02000d0000600a0b80000f229e000002554777b7
92313734322d39:1 may be snapshot: disabling access. See resignaturing section i
0:01:26:38.585 cpu4:1049)LVM: 4476: vml.02000d0000600a0b80000f229e000002554777b7
92313734322d39:1 may be snapshot: disabling access. See resignaturing section i

Warning: This solution might not be a 100% safe solution so it's recommended to use your SAN admin  tools to take a snapshot or lun copy before doing these steps.

Warning2: We have seen similar behavior after running out of disk space on a VMFS LUN, but the steps below have not  been enough  to get access to the data again.

In most cases this situation can be resolved by doing the following steps:

1. Choose the ESX Server in viclilent and Click on Configuration / Advanced Settings.
2. Click on LVM -> LVM.DissallowLVMSnapshot 0 -> LVM.EnableResignature 1
3. Then click OK to apply settings.
4. Click on "storage adapters" and click the "rescan" button (upper right). That should let you see the volume again.
5. Right click the vmhba (under the controller adapter for your machine) and click "rescan".
6. Then the volume should be visible.
7. Then when you go to the summary tab, you right click and click "refresh" and you should see your storage1 volume (or whatever you changed it to).
8. Choose the ESX Server in viclilent and Click on Configuration / Advanced Settings.
9. Click on LVM -> LVM.DissallowLVMSnapshot 1 -> LVM.EnableResignature 0
   
10. Click OK to apply settings.
11. The name of the visible LUN is now called something similar to snap-5bcf31f1-something. You can now rename it back to what it was called originally. If you have several ESX hosts accessing the LUN you might need to mask out other servers from accessing this LUN in order to rename it.

This situation has been observed on several different ESX 3.x versions.

References:
http://www.vmware-tsx.com/download.php?asset_id=49
http://activeanswers.compaq.com/ERC/downloads/4AA1-0820ENW.pdf
http://communities.vmware.com/thread/72948
http://communities.vmware.com/thread/60710
http://communities.vmware.com/message/868316

Thanks to Cody @ http://professionalvmware.com/ for pointing out the renaming of the LUN after this operation.