Are the good deeds performed prior to abstaining from prayer for several months or a year nullified, or do they remain valid after repentance?

Neglecting prayer is a major sin that nullifies good deeds, as stated in the Hadith (definition): "Whoever misses the 'Asr prayer, his deeds have been nullified." However, one who repents from a sin is like one who never committed it. So, if you sincerely repent, the reward for your good deeds remains yours. The condition for repenting from neglecting prayers, according to the majority of scholars, is to make up for the missed prayers.