My Mandevilla was doing great until about a month ago. Now all of its leaves have dried and fallen off and I have been left with is a plant with no leaves or flowers. I have had some aphids on my green pepper plants (second question. . . big leaves, but not flowering). Can either of these plants be saved? Leaves on underside of pepper plants are spotted with brown spots. As for the mandevilla, can I trim it down and have new growth?
If the mandevilla stem is dry and brittle, I would cut it back to the point where it seems to be flexible. Hopefully, it will resume growing. This link might answer more mandevilla questions: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
Your peppers may have bacterial leaf spot. This link will take you to a group of articles on bls: http://www.google.com/cse?cx=012078378210706707791%3Af1h5n_k1r5e&ie=UTF-8&q=bacterial+leaf+spot&oq=bacterial+leaf+spot&gs_l=partner.3...80037.80037.0.81220.127.116.11.0.0.0.253.253.2-1.1.0.gsnos%2Cn%3D13..0.0.0..1ac.hKIbY0Qz4a4#gsc.tab=0&gsc.q=bacterial%20leaf%20spot&gsc.page=1
Hopefully, the following article will help answer your question about peppers not blooming: https://www.gardeningknowhow.com/edible/vegetables/pepper/pepper-blossoms-falling-off.htm