No summer funding, what to do during the summer?
Depending on your specific field, and when you've found out summer funding isn't guaranteed or available, I present main options in no particular order:


Research internship in 'industry' ($ to $$$$$)
Apply for summer grants to work in another University in different, but hopefully related, research ($ to $$)
Get a summer job or non-research internship - ideal if it uses some set of related skills to your PhD, but if not, oh well, bills to pay ($ to $$$+)
As Dawn suggested, a sub-set of summer job is to seek the option to be an instructor, summer teaching assistant, or grader in your University (or a nearby institution), or even being an adjunct for the summer if that option is available to you ($ to $$)
Vacation, travel, relax rejuvenate, try some hobbies, spend time with friends and family (-$ to -$$$$)
Focus on your own personal research, free of official obligations (at least -$, due to living expenses)
Have an existential crisis due to the lack of driving structure which otherwise distracts you from your morality/mortality and the uncertainty inherent in life itself (free, and can stack with other options)
Choose your own adventure


Above all, I suggest planning ahead - if not for this summer, then for the next one. Most outcomes are uncertain, so judicious planning for multiple applications, budgeting (so you have options on how to spend your summer), and trying to develop a number of options will be important.

Note also that many of the options don't need to - and perhaps should not - take up the entire summer, so you should have the option to try out multiple things. Remember: academic life is a marathon, which extends off into the unknown future until you fade away (go Emeritus) or leave ("go into industry"). That makes the longest ultra-marathon on earth seem short by comparison. No matter what you do, try to change things up a bit, work in some rest and contemplation, friends and family, and a bit of fun for yourself too. If academic life starts to resemble a 24/7/365.25 prison sentence, you might want to reconsider your worldview.