Which fonts? Adobe Fonts?
Not Adobe fonts. Fonts I've purchased. Have always worked before.
You can try to create a folder with the name Fonts in the Illustrator application folder and put fonts in there.
On the Windows platform I have seen some glitches with some fonts not showing up in one of the past version builds of Illustrator. One type family called Sica (by dooType) had some weights not showing up. I haven't noticed an issue with the current version of Illustrator. Likewise, I've seen similar font disappearance glitches, but with different fonts, in other applications such as CorelDRAW. For example, I have a 60 font super family called Vito (originally sold by Typejockeys, but now by Dots & Stripes type) that works fine in Illustrator. The fonts come in 5 widths, 6 weights per width and with matching italics. None of the 5 bold upright fonts show up in CorelDRAW 2020; they only appear italic. They do show up properly in previous versions of CorelDRAW.
Anyway, in the case of Windows, one of the culprits may be the Windows OS itself. Apparently the OS thinks there are only four weights of any given typeface (regular, italic, bold and bold-italic). When type families greater varieties of weights (from hairline to ultra black) the OS sometimes makes a mess of the situation. Type designers have to do things "under the hood" to fool the OS into working with the fonts properly.
With type technology continuing to make more leaps forward, with things like Variable Fonts or SVG Color Fonts, the folks at Microsoft and Apple really need to get on the ball at properly supporting current standards as well as these new standards. It's either that or we're going to end up stuck going in a time machine approach and become more reliant on third party font managers or type utilities like Adobe Type Manager from 20+ years ago.
Sorry to hear about your experience. I'd request you to please try troubleshooting suggested on this help article(https://helpx.adobe.com/in/fonts/kb/troubleshoot-fonts-illustrator-indesign.html) & let us know if it helps.