When it happened to me, I just thought, oh well, & took care of myself right in front of him & low & behold, he revived. But that was with a younger partner and we'd already done it several times when it happened.
But I'm sure if it happened again, I'd just have us do other things that don't require an erection for me to get off. Maybe that's because I've been with women & we had to make due without an erection, so I'm well versed in alternatives. As long as he still showed interest in continuing, I'd be happy.
Hey, it's not like men are robots. Things happen that aren't in their control. If it's the first time this has happened with that person, sure I would start to doubt his interest & be disappointed, especially if it signaled the end of the fun. But I'd certainly try & figure out why it's happening.
As you get older, you learn to expect there are times this will happen. And as Gunsmoke said, it's not necessarily because he's not interested. So you learn some tricks to work around it. If he assures me that he's still there mentally & tries other ways to stay in the game, then I would be happy with that.
Just look at all the tools like penis pumps & cock rings you can use from EF, not to mention all the dildos & vibrators. Along with hands & mouths, there are so many other avenues to pleasure each other, occasional loss of an erection is not the big deal it's sometimes made out to be.