Just sharing this in case anybody else runs into this issue, since I couldn't find any posts about it online and it was a huge headache to actually determine the problem.
I turned on my Deck the other day after about a month of not using it, and it would hang on the login screen forever. After some troubleshooting, I finally noticed that the system clock had somehow desynced. This was causing an SSL certification error and I couldn't connect to the steam servers or any other website. I was able to get into desktop mode, but because you can't use the on-screen keyboard without being logged into steam (something Valve really needs to fix) there was no way to type in my password and change the system time. Luckily though, you can boot into the bios and just change it directly from there, no password needed.