What if you are in the middle of telling a joke or story and forget the punchline?

I’m awful at telling jokes – mostly because I usually forget the punch line right in the middle of the delivery. What I'm doing is.....
Michael Jackson? Thats all I gotta say. ...He's become a punchline. He has! Michael Jackson is a punchline. To any joke you want. If you ever forget the punchline to a joke, just say 'Michael Jackson.' "Two Jews walk into a bar...Michael Jackson!" "Why did the chicken cross the road? Michael Jackson!" "So the farmer brings his daughter to the dinner table--Michael Jackson!" It works for anything!

I'll say something like, "what's that over ther?" And once they look in the opposit direction, ill jump in the bushes virtually disappearing into thin air inpressing them so much that they forget about the joke.

Simply say to the audiences, to be continued...
It's better than to come up with an alternate ending since it's less effort. You don't need to think anymore. You don't need to search on the web too.

I'd make a joke about the fact that I forgot!!! Something like..."They say the mind is the first thing to go. I say it's the 2nd thing!!!"

The human race has become an awkward mess. Why would it be embarassing if you forget the punchline? just say you forgot it but it was really funny or something. Its not a theisis defense for christ's sake

I'm perfectly capable of coming up with a new punchline. I'm thinking that many new jokes started that way.

As you've already anticipated this scenario, then have an exit-strategy joke pre-prepared. Jokes mostly end in punchlines and the chances of forgetting one is far less costly than thinking up an exit-strategy joke. Or just make it a habit to mentally snapshot the punchline before you invest time and later embarrassment into setting it one up!

if you're are in ther middle of the joke and forget the ending just make something up and start laughing

like the old joke no soap radio

I would say "Well, this is awkward - I forget the punchline". When everybody just begins to laugh, start right into another joke.

It would an embarrassing situation if one forgets the punch line.

Without the punch line it would be a rather dull joke but do not cut in the middle of the sentence. Make something up and move on.