Hair dye hair loss horror story - will hair go back to its original thickness?

While chemical processing can damage the hair shaft, cause breakage and texture changes, it does not have any effect on the underlying follicle or hair growth. Hair may break near the roots and the individual strands my feel coarse or fragile but it won't cause true hair loss.
Hair generally grows about 1/2 inch per month so if it's been about 4-5 months since you dyed your hair, the 2 inches nearest your scalp should be fresh growth and indicative of what your normal hair quality and thickness is. If this hair is different than what you remember, it may be because of other changes (such as telogen effluvium from stress or other factors). A trip to the dermatologist may be a good idea. I hope this answers your question