Repairing *really* damaged hair?

I've been incredibly stressed out lately, and my hair has just decided it would be fun to fall out. Eurgh. Is there a simple remedy for this? Any possible way I can thicken it... or something? It's driving me crazy.
Update: Thank you for all this advice! I'll just add I don't straighten my hair, blow dry it or use products. I just wash it and tie it up for school.
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