datastore inaccessible

[#1]

One of my datastores is now greyed out saying its inaccessible in brackets next to it. I notice there is no space left on the datastore either. It has 3 VMs on the datastore. I cant move the VMs to another datastore or do anything. The only options I have when I select the datastore is to unmount it. 

What can I do to keep either clear up space and get the datastore back working? Or will this datastore never work again? How do I get my 3 VMs onto another datastore If I cant use the current datastore?

[paudieo]

I assume this is a VMFS based datastore?
if so as per  https://kb.vmware.com/s/article/1003412 you need at least 200MB free on your VMFS datastore to keep it from behaving unpredicatabily 

Also When a VM is powered on it creates a swap file stored , usually, with the VM
This file has the size equal to the VM memory, thus may use space on the datastore

Before attempting anything you need to carefully examine the log files on the host with the inacessible datastore (  /var/log/vobd.log or /var/log/vmkernel.log )  to understand if the full datastore is the root cause of why its inacessible 

you can also check for other space eating causes such as old VMs that are un-registered or VMs running on snapshots

Make sure you have backups incase you need to revert