Macbook sound won't turn on

I updated my hardware recently in my old white macbook. Not sure of the spec, but it was the last white macbook apple made. Maybe around 2009/10? Anyway, I got a SSD for it, and now it's working really fast.

It was working really well until earlier when I muted the sound as I was in the library. However, now when I try to turn up the sound, it won't let me. Just has a circle with a line through (like a no entry sign) come up where the sound level used to come up, and now it won't work at all. Is there any way I can correct this?

Thanks in advance.