Getting a laugh out someone is usually the quickest way to cheer them up and this can be done by reminding them of a funny memory, offering a comical compliment, or assuring them that things could be worse.
If you have known each other for a long period, it can be easy to remember a funny moment you two have shared. Relay the story to them in great detail and remind them how much it made you laugh in the moment. You can also think of a way to compliment them and make them laugh at the same time, such as, "you're cooler than a cucumber." Another way to make them laugh is to think of a crazy scenario that would be a worse situation, like: "It could literally be raining cats and dogs right now."