An existing snapshot is no longer valid and cannot be mounted for subsequent operations(1542)

from a command line:

bpfis query

This displays any acive snapshots created by bpfis

You can then use:

bpfis delete -id snapshot_id_found_above

but you need to try and find out why it happened

also check VSS  using "vssadmin list writers" as  you may need a reboot anyway