KDE plasma desktop totally failed; can't log on

Everyone: Has anyone had a problem like this? For the past several weeks, one of my two F22-KDE machines (a Dell Inspiron 1200 running F22 in the KDE spin) has been freezing up while trying to bring up Plasma and two auto-starting applications. Eventually I could get to a good session, but only after having to log out and log back in once or twice. And sometimes the system would freeze up for no good reason, and I would have to do a hard shutdown--holding down the power button for five seconds. Yesterday, during the most recent big update push (160+ packages), that system froze up in the middle of the update. I have no idea what it was trying to do at the time. I do know it failed to install the latest kernel. I then used the power button to force a shutdown. Ever since then, whenever I try to boot that system, it will boot, all right--to the last kernel before the new one. I get to a login screen. But when I try to log in to a Plasma desktop session, I get a brief message, too brief to read. And then the login screen returns. Result: I cannot start a Plasma desktop session. But: if I designate the Session Type as a Failsafe session, I can get in. I then arrive at a command prompt. Now that would let me do something. I want to know what. And I wouldn't even know what logs you'd need to see--nor how to get them off that system and onto a system that can send e-mail. In sum: I think I can rescue that system. I just don't know how to do it, once I log in to a command prompt. Temlakos