Custom Playlist Disappears

Hello all. This has happened to me twice already and I have no clue as to why. I would spend time going through my entire music library on my iPhone to create a collection of songs for my own playlist, only to find it has disappeared. Instead I find random playlists that appear to be defaults (i.e. 90's hits, most played ect). I tried to sign off and on to my iTunes account. I have also tried to reboot my phone but it is still not there. It's very frustrating and if anyone can help remedy the issue or avoid it in the future that would be great.