I fixed it.
First I tried re-installing java jre 8 291. This did nothing.
Then, I went to command prompt and tried running java -version and got an error that a ms-win-crt-runtime-l1-1-0.dll was missing. Googling this lead me to here which said that it had to do with an outdated C++ Redistributable.
I installed Microsoft Visual C++ Redistributable 2015, and then Ignition started working again.
As to how things got into this state, I have no idea, but this was the fix.