My mom now 90 had hearing loss at young age but could hear a little and used hearing aid for 20 years now she has suddenly lost all her hearing why?

Few things... She might have an wax impaction in the ear canal - which can happen with hearing aids. Usually sudden hearing loss is from trauma, sudden loud noise or rupture of the ear drum. I'd get this checked out by her doc - they'll do an exam looking for wax impaction, eval the ear drum and likely do a hearing test. Good luck!
Hearing loss. Your mom may have already sensorineural deafness and it appears to have worsened resulting in total deafness. It can happen . Try adjusting/ recalibrating the hearing aid or have her ears checked for wax build etc.