Fan stuck......either due to shaft binding/dry bushings, debris/nests in blower cage OR start/run capacitor bad
Remove ceiling grill to expose fan wheel and then with t-stat OFF turn wheel by hand...then try turning A/C On via t-stat (sometimes just from lack of use they will need a helping hand...dry bushings)
If fan still doesn't then probably the start/run capacitor.
Roof top.remove shroud....then look for cover plate on side of 'metal shroud'....inside is where they will be located
Could be a couple 'electric canisters' ---one for fan & one for compressor
CAUTION: they can SHOCK you due to stored voltage-----need to be discharged