Salvaging a plasmid from a cell culture stored at the incorrect temperature
I would rinse the cells off the agar using a small volume of LB or similar rich medium. Then use a small aliquot (10 &mu;l) to inoculate a 5 ml culture under selection to see if anything will grow. Meanwhile the remainder of the cell suspension from the vial can be used as input for your favourite miniprep procedure.

If necessary, the DNA that you recover should then be used to transform a suitable E. coli strain. Don't bother trying to analyse the rescued DNA first since polysaccharide contaminants from the agar will probably inhibit any restriction enzyme that you might use.

Good luck!