Do not start a rebuild
A rebuild writes to every disk. If a second disk is marginal, the rebuild can push it over the edge and overwrite parity you still need.
Do not reorder the disks
Label each disk with its slot number before removing anything. Order matters for reconstructing the array.
Do not run repair tools on the volume
CHKDSK and fsck can make a corrupted file system worse. Image first, repair the image.
Do call before you power-cycle
Tell us the controller, RAID level, disk count and what happened. We can often advise within the hour on how to keep the array stable until it reaches the lab.