Don't be discouraged
You probably did do asay with most of your muscle mass, but the good news is, that is reversible!
If 5 pound weights are too heavy, go get some 3 pound weights to start with. I bet in less than two weeks the five pound weights will seem easy
I'd go get 3,8, and 10 pound weights. They are inexpensive, and even as you get stronger, you'll probably still use the 8's and 10's. I'd get a 55cm. stability ball, too. They are really handy for doubling as a weight bench, doing ab and hamstring exercises.
Start with a simple lifting program of 1 exercise per body part, at a weight where you can finish 1 set for 10-12 repetitions. For the first week, I'd aim for two times a week, then add a second set for the second week. By the third week, you can probably experiment with higher weights, more and different exercises, and eating a little more.
Here's a real pared down list of exercises I'd suggest for the first week (and the body part each works):
dumbbell squats (quads)
dumbbell flat back deadlift (hamstrings and lower back) or lying hamstring ball pull
dumbbell row (mid-back)
dumbbell chest press (chest)
dumbbell flyes (chest)
any variation of curls (biceps)
tricep kickbacks (triceps)
Aside from weights, there are plenty of body weight exercises that accomplish the same thing; pushups or modified pushups work the same muscles and doing chest presses and no equipment needed. You can do dips for your triceps on the edge of a chair or coffee table...walk up and down stairs carrying gallon jugs of water...resistance bands are inexpensive and useful for a lot of exercises.
After about 3 weeks of that, you'll definitely want heavier weights and new exercises!
To see what these exercises look like, check www.exrx.net
. Another great source of info is www.stumptious.com
You can re-build your muscle mass, but you need to feed it to make it grow
I think you'll be pleased and surprised at how quickly it comes back with proper food and exercise.