Absolutely NONE of my terminal commands are working: cd, ls, nano, pry, flutter, python3… Does anyone have any idea what’s going on?

I can’t think of what’s recently changed, besides perhaps updating to a new version of Mac OS Catalina (10.15.2). All of these commands have been working for me up until today. First it was pry that stopped working, but everything else was fine. Now, they’ve all stopped working! Has anyone encountered/solved a similar issue?