How do i get back in shape?

i used to be kinda chubby back in the 8th grade 180lb peak 5f6in tall i lost some weight over time throught time with alittle workout and alittle diet i got down to 135 after 3 years and i was in my prime i mean i was skinny but kinda ripped ..and over the winter i injured my knee so i stop running and i diet hard to compensate now im 124lb 5.7 skinny but alittle flabby low energy low labido for goddness sake im 20 ..i want to gain some weight back more like mussel mass how much should i eat? calories im by the way 5f7in and 124 lb and look kinda in shape i want my energy back ..and i been trying to cut my carbs and fat can that be the problem?