sda1 (?) raid failed on debian - what to do now?

Don't panic, this is a common and recoverable error. Your hosting company set up a two-disk redundant array to protect the data in case one of the disks fails. This failure has now occurred. The output indicates that sda1 has failed, and that the RAID1 array is working, but degraded.

Right now you're on borrowed time, though. If the second disk fails, that data is gone and you'll have to restore from backup. Ask your hosting company to replace the failed disk immediately and get back to you when it's done!

Also, making sure the backups are fine before messing or having someone mess with a degraded raid is a good idea. Cascade failures sadly happen, and so do mistakes by host staff (triple check that you and the hands at the hosting company are on the same page as to what is to be done to which disk).

AFAIK, if the device is called /dev/mdX, it is always linux softraid, so no hardware storage controller apart from a straight SATA or SAS host adapter is involved.

There are ways in linux to tell it that a disk is to be logically removed or has been added, however these should ever only be necessary if hotplugging directly attached PATA or parallel SCSI devices (which should be considered verboten anyway on hardware that does not explicitly support it anyway).

smartctl (from the smartmontools package; do not run it if there is an ssd involved AND the provider did not set up a smartmontools daemon or cron script on the server; there are versions of smart utilities known to damage certain ssds) can tell you a lot about WHAT is wrong with a drive, especially if directly attached as is the case here, so can calling dmesg. The spinup_count and power_on_hours values you get on the replacement disks you get sometimes make for interesting discussion topics with hosters ;)