How do I get over year and a half composers block?

I am a composer who has completed two symphonies, six sonatas and various other pieces of music and my ideas usually just hit me out of nowhere in complete form and i just wrote them down without much correction. but over the past year and a half nothing has come to me and I can't even force myself to come up with anything good I am losing self confidence by the day and am getting very depresses for not being able to execute something that has been a natural talent since i was 9. And now my school wants me too write a 3 movement concerto for the next orchestra concert in april which I accepted because I thought it might give me some motivation but still nothing. I am on the brink of giving up my long time pion which I always thought I would never have to do, Is it possible I could lose a talent like that so fast and if so could I ever reclaim it. Have any of you other orchestral composers gone through this and if so how did you get out of it I am desperate for advice please help me I need to complete this concerto and gain my self confidence and talent back.