What is it clogged with? Our parking lot drain used to get clogged with leaves. We used a broom to clear it out.
Try a snake. You might need a really long one.
HI there, Borrow or Buy a set of Drain Rods. Connect them up and see if you can shift the blockage that way......
Rent a snake from a tool supply. I clean mine out every spring. My lazy and money to burn neighbors hire plumbers or landscapers to do this.