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Stress/depression can cause decrease in libido.
You may want to rule out medical causes for this (having a hormonal panel test and checking for prostate issues)
If you think that the cause is psychological, the stress level has to be managed (whether with counseling, hypnotherapy, exercise, meditation, etc)
There are some options that can increase the testosterone level- Maca, Tribulus, Macuna Pruriens. There are also supplements that can target muscle building (especially if you exercise) and release of GH.