Hi, Aravisian,
Thanks for your help and for these links. Could you please let me know whether there is a link for reporting bugs to the Zorin Group? (There's a launchpad online, but it doesn't appear to be maintained.)
I'm wondering whether the problem that UselessBoi and I were having is related to the problem with kernel 5.11.0-34. I realize that the kernels are different -- my problem seems to have been caused by kernel 5.11.0-43 -- but the symptoms are similar: black screen (connected with restarting, shutting down, suspending). My CPU is AMD A9-9420 Radeon. I've been able to upgrade to kernel 5.11.0-40 (per the solution to UselessBoi's post), but nothing higher than that.