I installed a nightly or unstable build of a custom ROM and now my phone doesn't work I installed a nightly build, but now some important features of my phone don't work: it boot-loops, or can't make calls, or can't use mobile data, or something like that. What can I do about it?


  N.B. This is a "canonical question" for all similar questions, because questions about specific bugs in one particular nightly build of a ROM are way too specific to have any long-term value. See this meta question to learn why this question was created, and why your particular question about nightly builds might have been closed as a duplicate of this one.
As our cyanogenmod tag wiki explains:

Nightly builds are created each day, and represent a snapshot of development, with minimal testing and quality control. They often have missing features or critical defects, changing from one day to the next, and are not recommended for everyday use. If you ask a question here about unusual behaviour in a nightly CM build, we'll probably suggest you switch to a more stable ROM. Experimental builds are the same as nightlies, but are one-off special builds outside of the nightly routine.

That quote had a lot of words, so let's repeat the crucial bit.
If you want to use your phone, do not install a nightly build.
If you already ignored that recommendation and installed a nightly build, and your phone doesn't work, stop using the nightly build. Reinstall the latest stable, release candidate, or milestone build for your phone, or the stock ROM for your phone. If that means going to an older Android version, you probably need to wipe userdata (i.e. factory reset) at the same time.
If you don't know where to find a stable version of the custom ROM, or the stock ROM, check out Where can I find stock or custom ROMs for my Android device? The exact steps to install a ROM are different for each phone, but if you already installed a custom ROM, you probably already know how to do it for your phone. If not, just search this site with your phone model, and you'll probably find step-by-step instructions.
You should probably also report the bug to the developers. They might not be interested if it really is a nightly build. ROM developers don't expect the nightly build to work. But if it's an experimental, unstable, milestone, or release candidate build they probably will want to know. If the ROM in question is an official LineageOS port, you can report a bug using their Jira bug tracker. More information about their bug reporting process can be found on the LineageOS wiki. If it's an unofficial Lineage port, or different custom ROM, you probably need to contact the developer directly, or check the forum post you got it from.
In particular, if you installed a nightly build and it broke something, this site is not a good source of help. Generally, the only fix for the problem is to install a different (working) ROM build, so that's all we'll ever tell you to do. If there's some workaround for the bug, maybe only the ROM developer knows it, and the best place to get their advice is by the means above: the bug tracker, or whatever forum they hang out at.
Maybe you installed the nightly and it actually broke your phone hardware, or installed a new bootloader that can't flash images any more. If that happened, tough cookies. You played the lottery, you lost. Nightly builds can do all these things to brick your phone, and more. That's why even the ROM developers advise you not to install them unless you're willing to take that risk.
If there's only a nightly or unstable build for your device, you might read Why isn't there a Cyanogenmod stable ROM for my device? to learn about why porting ROMs is hard, and what you can do to help accelerate development of a stable, usable ROM for your device. But until then, you'll have to either stick with a different ROM, or just take the lottery of not knowing whether tonight's upgrade will finally be the one that bricks it.