Picture folder disappeared from gallery on Huawei Ascend G6
What the problem is, is probably this:
Your clean master probably enlisted your 500 pics as worthy of cleaning, thus deleted them all in the process. Its just a thought, but its likely to happen.

I use ES Explorer to clean stuff on my phone, and I verify every single thing it plans to delete everytime I'm about to clean. You never know, it might include your wheat among the weeds!

Another possibility is what happens when the storage is almost full. Happened to me once, and witness a friend face same trouble. Storage was almost full, phone restarted, and everything personal info was gone, from pictures to videos to everything I put on. Perhaps one of those nightmares, but only happening to images.