What are potential causes?
I don't believe it's a mental problem, or one related to depression. I have a pretty healthy mental outlook. However, I have definitely felt zero sex drive over the past couple of months. I can't remember the last time I had morning wood either.
Ok.
Full stop.
I am speaking to you as a mod and as an endocrinologist.
If you are experiencing rapid testicular atrophy, that is a red flag. It indicates you most likely have a central (read: pituitary or hypothalamic) problem. The other possibilities include infiltrative disease (infectious, deposition, degenerative) which may affect either the gonads or central hormone producing elements.
This can be the sign of a potentially life threatening issue, because your testosterone level may not be the only hormones effected, and you may have acclimated to other symptoms since the onset was gradual.
STOP asking for advice on this forum. You cannot fix this with diet or exercise. Do not take herbals or try to obtain testosterone supplements, because it does not resolve the central issue which, as stated, may be serious, especially if you delay.
GO see a doctor.