New to the sketch thing

Hi im fairly new to writing sketches as I write more stand up but ive had a eough idea for a sketch butsone mates think it would be crap and some mates think it could be funny...and I think maybee ive subliminaly seen something like this before somewhere else???? Anyway heres the idea....its not really scripted...like I said just an idea atm.

Two men go running down the corridor,run upto a woman and ask 'whats happend? Is dave ok?'. The woman replies 'yeah hes going to ne absolutely fine he just had a bump to the head.'  One of the men says 'well at least hes ok, can we go in to see him?.the woman then replys 'yes you can but they gave him some medication and he won't come round for about another then minutes'    one of the the men replies 'cool we can be there for when we wakes up'.

Next scene. ..the two men in a hospital room waiting for there friend to come around from the medication/bump to the head.

The friend slowly wakes up confused and in a daze, looks at his mates and asks 'whats going on?'.

One of the friends then replys   'mate I don't wanna worry you and don't pacic....but its 2018 dude, youve been in a coma for five years..

Perhaps I'm being a bit stupid, but it doesn't seem to be a joke.

Yeah maybee I could have explained it better lol basically the guy in hospital has only been in ther a few hours but as he was unconcios so the friends lie for a laugh when he wakes up and make out that hes been in a coma for a long time...but he hasnt lol

That's not a sketch, really. That sort of comedy only really works if you have some interest in and knowledge of the characters - ie, the scene could work in a sit com, potentially.

My riule of thumb is, if you need to build a realistic world for the joke to work (scenario, relationships between the characters, and so on), then it won't work as a sketch. If your scenario is based on a single premise (the CEO of a company is a dog, a policeman has an irrational fear of the number 999, or whatever), then it will.

Other, less ontological methods are available, I should stress.