First you should try to determine what you mean by "virtual machines disconnection":
- was it unreachable from anywhere?
- did it crash or was suspended?
Start your analysis with the vmware.log of the VM at the time the issue occurred - these are located in the VMs namespace directory (e.g. /vmfs/volumes/DatastoreName/VMname/vmware.#.log).
Yes you are right , my virtual machines some of them crashed and some of them first get freeze and after that going to disconnected.
You should be able to browse to the datastore that holds the VM files and there should be a log file in there which you can download and read
It would also be good to check through the guest OS's event log to see if it's OS related. Also, it may be worth checking the performance metrics of the VM shortly before it crashed to see any spikes in memory/cpu etc