trouble gaining muscle and keeping it 😞

Mimi😽
Hi friends,
I've been going to the gym for about six months now and I did see some improvements but overall going up in weights hasn't changed much at all including all areas of the body. I'm naturally pretty lanky and I joined a gym to gain some definition and become stronger. 
Today felt like a major setback because when I went to do my leg machines after not being in the gym for about a week I could not lift even half of the weight I had been only a week ago :( I just don't understand it. I started having knee pain and I went home and cried because I felt like this whole time has been a waste. (There was nothing wrong with my form or the way I was using the machine).
I know this isn't something to be rushing and I'm not, but I'm just overall very disappointed with my progress. 
I recently purchased protein powder and creatine in hopes that will help me out.
Any advice on how to stay motivated and become stronger? 
Thanks