Hi. A full body training split as its called is required to regain your loss muscle, I will post what worked for my mate who had a similar circumstance to you. It is mostly easy and quite quick however you do need access to weights so a gym membership would be very beneficial.
This program comes in two parts, (A) and (B). Rest is very important so it is best for 1 week to do (A), then rest the next day, (B), then rest the next day, (A) then rest for two days from doing that split. The following week you would then do (B) instead of (A) and (A) instead of (B). It would look like this as an example
Monday (A)
Tuesday rest
Wednesday (B)
Thursday rest
Friday (A)
Saturday rest
Sunday rest
Then the following week
Monday (B)
Tuesday rest
Wednesday (A)
Thursday rest
Friday (B)
Saturday rest
Sunday rest
I suggest starting at the barbell weight which an olympic size barbell weighs 20kg (45 pounds) and try to increase each week. I also suggest doing 3 sets of 8 reps for each as opposed to 5 reps as this ensures you do not try to go too heavy in the state that you are in, as the right form is required for each exercise.
You should also make sure to stretch and warm up properly by doing a light jog, jumping rope, anything that puts your heart rate up a bit for a few minutes. Doing this workout for 6 months consistently will see you improve your base strength dramatically. Your base will also become strong enough that if you wish to get into more difficult exercises that you would have no difficulty in stepping up to do.
(A)
Barbell Squats - 3 sets of 8 reps with 2 - 3 minutes rest between each set
Barbel Benchpress - 3 sets of 8 reps with 2 - 3 minutes rest between each set
Barbel Bent over rows - 3 sets of 8 reps with 2 - 3 minutes rest between each set
(B)
Barbell Romanian Deadlifts - 3 sets of 8 reps with 2 - 3 minutes rest between each set
Lateral Pull down - 3 sets of 8 reps with 2 - 3 minutes rest between each set
Seated Dumbbell shoulder press - 3 sets of 8 reps with 2 - 3 minutes rest between each set
Situps - Try to start with 4 sets of 20 situps and work up from there. If your core is quite good you could try resting a weight plate on your chest for extra resistance.
I recommend searching for videos from youtube for the right form. A good place to start is both Scott Herman Fitness
https://www.youtube.com/user/ScottHermanFitness
Scooby is another great natural lifter who specializes in home workouts for broke teens and also for older people as he is in his early 50's himself. He also has a fantastic website and believes in healthy vegan eating etc.
https://www.youtube.com/user/scooby1961