It's one of the most common complaints about free proxies: it worked five minutes ago and now it doesn't. There are a handful of predictable reasons.
The server went offline
Most free proxies run on personal or misconfigured servers rather than dedicated infrastructure, so they can go down at any time without warning. This is the single most common cause and there's no fix besides picking a different proxy.
It got overloaded
Public proxy lists are used by thousands of people simultaneously. A proxy that handled your request fine a moment ago may now be swamped by other traffic and timing out under load.
The target site blocked it
Free proxy IPs are well known to anti-bot systems since the same list of addresses gets scraped and reused constantly. A proxy can work fine for one website while being completely blocked by another that already blacklisted it.
What to actually do
Keep a checker handy, expect a meaningful chunk of any free list to be dead at any given moment, and treat 'grab a fresh one and retry' as the normal workflow rather than a sign something's broken.