What do you do if after 6 years of being an assistant professor, you come up for, and are denied tenure?
How hard is it to get a job in industry, when you are middle aged and have no relevant work experience? How hard is it to get another job in academia? Who wants to hire someone who was denied tenure at their home institution?
As with anything, it depends:
How hard is it to get a job in industry, when you are middle aged and have no relevant work experience?
Is that really true ? It of course depends, but academics even in very theoretical disciplines have skills of value to industry (witness for example the number of math PhDs getting jobs on Wall Street). Of course, if one presents oneself as having no relevant work experience, it's going to be very hard to find an industry job, but I question the premise. Maybe the right fit is not obvious, and maybe it will take some networking and talking to people to find the right niche, but it's more a matter of repositioning one's skills. It's easier said than done, but it's possible.
How hard is it to get another job in academia? Who wants to hire someone who was denied tenure at their home institution?
I know of many people who get faculty jobs after being denied tenure elsewhere. Universities don't operate on the same scale or with the same criteria or needs. It might need some time (again) to reposition oneself, and it might require some digging into the circumstances leading to tenure denial in the first place (painful, but sometimes yielding useful information).
It's by no means a definite thing, but it can be done. It will take some time though, also because without a job, one lacks leverage to dangle in front of another institution.
p.s The question was asked in the abstract, and the answer is thus necessarily vague. The real point is that every case is different, and there are always ways to move forward, if not exactly in the direction one is hoping for.