The quickest fix is to flood the cassette with solvent to free up the pawls that are stuck. Often they wedge up in the grease that has been exposed to water and turns into a thicker and harder gum.
Solvent (be it citrus, diesel, petrol, turps, etc) will dissolve the existing gunk and allow the pawls to be pushed out by their springs.
Once the solvent has done its job you want to let it all evaporate, then relubricate with something new. Don't be tempted to ride around without, cos it will rust up real quick.
Another very temp fix is to simply tap the cassette to try and make a pawl drop. if this works at all, its not a long term solution.
Personally I'd look at replacing the part, but you specified an easy solution.