Is it a problem with only one VM? is lt standalone ESXi or Cluster? What kind of Datastore are you using? Local Datastore or the attached one?
First, check basic things like Datastore free space, VM snapshots, is there enough CPU/RAM resources.
In case you have no time to dig deeper and this is not a PROD environment, simply restart your ESXi host, this is a bit harsh of course but still the solution.