There will be no impact, any running VM's will simply be shutdown and restarted on the same host when the server comes back up. Maintenance mode just makes it easier to migrate your VM's to other hosts prior to shutting the server down.
From the help...
Using Maintenance Mode
Both standalone hosts and hosts within a cluster support a maintenance mode, which restricts the virtual machine operations on the host to allow the user to conveniently shut down running virtual machines in preparation for host shutdown. While in maintenance mode, the host does not allow you to deploy or power on a virtual machine. Virtual machines that are running on the host continue to run normally. You can either migrate them to another host, or shut them down. When there are no more running virtual machines on the host, its icon changes and its Summary indicates the new state. In addition, main/context menu and Command section operations involving virtual machine deployment are disabled when this host or its resource pools are selected.
DRS Clusters and Maintenance Mode
For DRS clusters, the automation mode determines cluster behavior when you enter maintenance mode.
A Fully Automated DRS cluster automatically migrates running virtual machines to different hosts as soon as maintenance mode is enabled.
Note: If no appropriate host is available, DRS displays information on the Tasks & Events tab.
A Partially Automated or Manual DRS cluster displays recommendations for migration of running virtual machines, with the recommendation type Host Maintenance displayed in the recommendation list.
