What happens, if a host fails and the VM's has no HA enabled?
That VM stays where it was, no HA no automatic recovery.
Can we start them manually on another host or are they blocked on the failed host?
If the VM itself is on shared storage and doesn't use any local resources of that node you can recover it by moving the configuration file in
/etc/pve/nodes/<deadnode>/qemu-server/<VMID>.conf
to another node (same sub directory).
This should only be done after one ensured the state of the seemingly failed node, as if only a single network went down, or the only the cluster stack, it could be that the VM still runs - and if you "steal" (move) and start that VM on another node you may corrupt the underlying disk images.
For this reason we do not allow a simple move VM/CT, if it's hosting node is unresponsive.
The HA manager ensures that the other node isn't running any VM/CTs by self fencing and aquiring the failed nodes HA locks, only then it recovers a HA service.
If you're failed node if offline for sure you can move and then start the VM or CT just fine, though.
If it died during some locked operation on the VM you need to unlock it manually (qm unlock <vmid>) though.
Hope that makes things a bit more clear