There are two kind of channel closure: cooperative and forced.

cooperative closure is the standard way of close a channel: one node initiate the closure and the other one "agree" to close the channel. Both node must be online for this

Forced closure is the "emergency shutdow". Both nodes keep a special transaction with the latest channel state (=i own X sats and you own Y sats if we close now). Only one node is necessary for this procedure and allow to close the channel, but:

- who initiate this closure pay higher than normal fee

- The sats of the closed channel are blocked for some days (=cannot be spend)

A malicious node can keep previous state of the channel for initiate a force closure where it can receive more than what own (=stealing for you), however, if your node is online (or a watchtower who is protecting you) while the channel is force closed AND the fund are still blocked, a punisent transaction will be used for force the malicious node to give to you the fund you really own

Backup in lightning is still a problem. To be simple, prevententiom is the key:

- You can backup your channel information (thunderhub and RTL make it easier) for kindly ask to the other nodea to initiate a force closure (because you probably have lost all channels information and are not able to do it anymore)

- you can use watchtowers as insurance, they will send the punishment transaction for you. LN+ have a section for find a watchtower with a mutual defense agreement. Public open watchtower may be overload this days. You can still have more than one watchtowers

- If you have the knowledge, put the data of your lightning node on a RAID-like storage. Never do spot backup of these data because restore a previous state of a channel can make you lost the sats of that channel