Which version/build of ESX(i) do you use?
How many snapshots do you see in the datastore browser?
If you have only one snapshot or if you have multiple snapshots AND you are running either ESX(i) 3.5 U5, ESX(i) 4.0 U2 or ESX(i) 4.1, I'd suggest you go with Gregg's suggestion to power off the VM, create an additional snapshot and then do a "Delete All". Depending on the snapshot size you should calculate with some downtime.
If you have multiple snapshots and you are not running the latest version of ESX(i) please DON'T use the "Delete All" button. This will most likely end up in a full datastore. In this case please post a screen shot of the datastore browser window, showing name, extension, size and time stamp of all files in the VM's folder.
André