At sudden i'm getting this error "This page isn't working HTTP ERROR 500 " in ColdFusion2016

It's rare that errors happen "all of a sudden" for no reason. So first, can you tell us if you know anything that may have changed in your environment? It may not be code. It could be configuration of CF, or the web server, or the box on which they run.

If you'd say "nothing" changed, then let us know if you see anything in ANY log (in the cf logs folder) at the time this error appears on screen.

Also, is this happening for all your pages and sites? or only some sites? As for the request that is now "failing", are you confirming that it was previously working? Until when? (And again, until "what changed"?)

Finally what you have quoted does not look like a phrase I've seen. Can you confirm that's exactly the error, since you have put quotes around it?