What are the terms of the grant that funded the summer internship? If you don’t know those, you don’t know if you were qualified for the job.
Nobody is going to hand you an internship. If there are 15 faculty in your department there are probably a max of 15 internships/research positions. How many CS majors and minors do you have at your school? That’s who you’re competing with. You can hang around waiting for an offer or take the initiative to go find one. If the first faculty member doesn’t have a spot, keep asking. And check with other departments too. You never know who might be able to use a computer scientist.